What Is SIP Trunking and How Does It Work

An AI agent that answers your main number still has to reach a real phone number, and it gets there the same way a call from your contact center does: over a SIP trunk.

This is not a new technology – it’s actually been around for more than 20 years, and telecom teams have run SIP trunks since ISDN days. It’s the alternative to the physical copper lines sold in fixed bundles, and while both technologies have been viable options until now, the latter is slowly being retired alongside ISDN circuits. 

So, if you’re looking to switch, or you’re just tired of paying for fixed lines you’re not using, SIP trunking is the way to go. It will continue growing and developing, and Mordor Intelligence puts SIP trunking at $85.07 billion in 2026, heading for $181.58 billion by 2031.

In this article, we’ll explain the basics of SIP trunking, how it works, the benefits over traditional phone lines, tactics on how to choose your new SIP trunking provider, and show you how you can combine it with a powerful AI voice agent. 

What is SIP Trunking? 

SIP trunking is a virtual connection that carries calls over the internet between a company's phone system (the PBX) and the public telephone network (PSTN), replacing physical phone lines.

Think of it as the phone line, minus the wire. Your PBX stays exactly where it is, and the only thing that changes is how it reaches the outside. Instead of copper lines from the phone company, calls travel over the internet connection you're already paying for, through a provider that hands them off to the public network.

The name comes from Session Initiation Protocol, which is the set of rules that starts a call, keeps it up, and ends it. 

The system contains three main parts: 

How Does SIP Trunking Work?

A SIP trunk links your PBX to an internet telephony provider, which turns each call into data packets and passes it on to the public phone network. 

For the call between your desk and the person you’re calling to work, you need four things: 

• Your PBX – the phone system you already have. It spots that a dialed number is outside the company and hands the call to the trunk.
• A session border controller (SBC) – a firewall built for phone traffic, placed at the edge of your network. It checks that the call is legitimate, encrypts it, and stops anyone hijacking your trunk to run up international charges.
• The provider (ITSP) – Twilio, Telnyx, Vonage, Bandwidth, and the like. They receive your call over the internet and have the carrier connections to hand it into the public network.
• The PSTN – the public phone network, meaning every phone number out there.

Put together, an outbound call goes like this: you dial, the PBX sends a SIP "invite" over the trunk, the SBC checks and encrypts it, the provider passes it into the public network, and the far end rings. Once someone picks up, the audio travels as a separate stream of packets in both directions until one of you hangs up. Inbound calls run the same path in reverse and end at whichever extension or queue the PBX chooses.

That last step is where an AI agent comes in. Instead of ending at a desk phone, the trunk (or just the calls you choose) can end at an AI platform's SIP server, so the agent picks up like any other extension. It’s how Synthflow works, and we’ve documented that exact setup here: connecting an existing trunk or PBX to an AI agent.

Is SIP Trunking the Same as VoIP?

No. VoIP is the broad category of calling over the internet, and SIP trunking is one specific way of delivering it to a business phone system.

People often use the two interchangeably, which is where most of the confusion comes from. In practice, they nest inside each other:

1. VoIP (Voice over Internet Protocol) – any phone call carried over an internet connection instead of a copper line. The umbrella term.
2. SIP – one of the protocols that sets up VoIP calls. The most common one, but still just the signaling rules.
3. SIP trunking – using SIP to connect the PBX you own to a provider. You keep the phone system and buy the connection.
4. Hosted PBX, or UCaaS (RingCentral, Zoom Phone, 8x8) – the provider runs the phone system in the cloud, trunking included. There's no PBX of yours to connect, so there's no trunk to buy.

So the choice comes down to what you have: a PBX you want to keep (SIP trunking), or a phone system you'd rather stop running yourself (UCaaS).

You'll also meet IP trunking, the older, broader label for any trunk over an IP network. Most IP trunks today are SIP trunks; some carriers use the phrase for a trunk on their own private network instead of the public internet.

What Are the Benefits Over Traditional Phone Lines?

In short, switching from PRI or ISDN to SIP trunking cuts costs and adds flexibility: you stop paying for fixed lines, scale one channel at a time, and get new capacity in minutes instead of weeks.

Lower Bills

A PRI charges for 23 channels whether you use three or all of them, plus the circuit and the hardware to terminate it. With SIP, you pay for the channels you need, commonly $15–30 a month each on unlimited-domestic plans (SIP.US, for example, lists $24.95 month to month or $19.99 on an annual plan), or a fraction of a cent per minute on metered plans. Mordor Intelligence puts the typical savings over legacy PRI at 25–65%, with long-distance and international calls falling furthest.

Capacity that Follows Demand

If you want to add a PRI, you have to order a whole circuit and then wait for an engineer to come over. With SIP, adding a channel is as simple as changing a number in a portal. Some providers drop channel counts altogether and bill on calls per second and minutes used (Twilio does this). That's what "elastic SIP trunking" means: the trunk grows during your busy hour and shrinks overnight, and you pay for the shape of your traffic, not its peak.

Calls that Survive an Outage

A copper line into a building goes down with the building, and there isn’t much you can do about it. A trunk, however, isn't tied to a location, so if the office loses power or internet, inbound calls can be rerouted from the provider's side to another site, to mobiles, or to a backup endpoint.

SIP Trunking vs a PRI Phone Line

The main difference between SIP trunking and a PRI line is that a PRI gives you 23 fixed physical channels, while SIP trunking adds or removes virtual channels on demand.

PRI is short for Primary Rate Interface. It's the circuit most business phone systems have relied on since the 1990s: a dedicated line from the phone company carrying 23 voice channels plus one for signaling (the "23B+D" you'll see on quotes). Need a 24th call? You order another circuit. Need to move offices? The circuit doesn't come with you.

The two compare like this:

The one point in PRI's favor is that last-but-one row. Because it's a dedicated line, quality doesn't depend on what else is happening on your network. That's why PRI still lingers where broadband is poor or a long contract hasn't run out. But carriers are switching those circuits off market by market, so the window for staying put is closing.

How Many Channels and How Much Bandwidth Do You Need?

As a rule of thumb, plan one SIP channel for every three to four office staff, and about 100 kbps of bandwidth per direction for each call happening at the same time.

Channels 

A channel is one live call, so you're sizing your busiest moment, not your headcount. For general office staff, one channel per three or four people covers it, because most of them aren't on the phone at once. For a support desk or sales floor, plan one channel for every one or two agents. If your current system keeps call logs, the peak number of simultaneous calls is the number to use.

Bandwidth

This standard codec for business calls, G.711, uses about 87 kbps in each direction once packet overhead is counted. Round it to 100 kbps per direction, so roughly 200 kbps for one two-way call. However, keep in mind that your calls share the internet connection with everything else in the office. If someone starts a big download or a video call, the phone audio can break up. QoS (quality of service) is a router setting that prioritizes voice traffic, so calls stay clear even when the line is busy. 

Example: A 200-person company with a 20-agent support team needs roughly 50 channels for the office plus 10–20 for the agents, so 60–70 channels and about 7 Mbps each way at peak.

One clarification, because providers use the words loosely: The trunk is the connection; channels are the capacity inside it. On elastic plans, the count can disappear altogether. Telnyx, for example, sells inbound channels from $12 a month down to $8 at volume, or bills per minute with no channel cap.

How Secure and Reliable Is SIP Trunking?

SIP trunking is secure when four things are completed: TLS encrypts the signaling, SRTP encrypts the audio, an SBC guards the edge of your network, and STIR/SHAKEN vouches for your caller ID.

An unencrypted trunk over the public internet is exposed, and anyone who can see the traffic can listen in, and anyone who guesses the credentials can make calls on your account. That’s already happening, and it’s a whole industry: the CFCA put global telecom fraud at $38.95 billion in 2023, about 2.5% of the industry's revenue, with PBX fraud among the top five methods.

Thankfully, it’s completely preventable, and any good provider will include the fixes above required to protect your connections. 

STIR/SHAKEN in particular is very important. Under the FCC's STIR/SHAKEN rules, carriers sign calls at three confidence levels: A, B, or C. Only A-level attestation says, "We know this customer, and they're allowed to use this number," and in healthcare, insurance, or collections, calls without this attestation are far more likely to show up as "Scam Likely."

Security covers who can get at your calls, but reliability is whether they connect at all, and that comes down to two things: the quality of your internet link and the strength of the provider's network behind it. This is why enterprise trunking contracts come with uptime guarantees, and why you should expect the same numbers from anything you put on top of the trunk, an AI agent included. 

Synthflow, for instance, runs its AI agents on an owned telephony stack at under 500ms end-to-end and 99.99% uptime, with data held in EU or US tenants depending on where you need it. 

How SIP Trunking Powers AI Voice Agents

AI voice agents use SIP trunking to reach the public phone network, layering speech recognition and language models on top of the same trunk a business already runs.

So the trunk itself doesn't change; what does change is where some calls end up. The PBX sends the ones you choose (after-hours support, appointment confirmations, first-line triage) to the AI agent's SIP address instead of a queue, and the agent handles them like any other endpoint, including handing a caller to a human with the conversation so far attached.

Two examples of what this looks like at scale:

• Freshworks embeds Synthflow’s voice AI in Freshcaller and Freshdesk, letting its customers automate 65% of routine voice requests, from routing and verification to after-hours ticket creation.
• A $230M BPO put 40+ branded AI agents live in 60 days on existing telecom and utility client programs. They now handle 600K+ calls a month with no new hires.

Synthflow is the AI agent layer in both cases, and it connects to the trunk either way you prefer: bring your own carrier (Twilio, Telnyx, RingCentral, Vonage, 8x8, and Zoom Phone are natively supported) or use Synthflow's owned telephony, which runs at under 500ms end-to-end. Either way, the whole call path is certified against SOC 2, HIPAA, ISO 27001:2022, GDPR, and PCI DSS.

What to Look for in a SIP Trunking Provider

We recommend the following: Weigh whether they own their network, how clearly they price channels and minutes, whether encryption is included by default, and how they handle porting and support.

The big names (Twilio, Telnyx, Bandwidth, Vonage, plus the carriers) all do the basic job. The differences show up in five places:

1. Own network or reseller. Resellers rent capacity and add a margin; faults take longer to fix when the problem is two companies away.
2. Pricing you can read. Expect a one-off setup or porting fee, a monthly charge per channel or number, and per-minute call rates (Telnyx, for example, from $0.005 a minute outbound). Model it against your own call pattern.
3. Encryption included. TLS and SRTP should be on by default. Telnyx and Twilio include them; some carriers still charge extra.
4. Number porting. The FCC requires single-line ports to complete within one business day; business ports with many numbers typically take two to four weeks, so start early.
5. Support and SLAs. A written uptime guarantee and a support desk that picks up at 3 a.m.

One more filter if AI is on your roadmap: Check that the AI platform you're considering accepts any SIP provider, so the carrier decision stays yours.

Can You Build Your Own SIP Trunk?

You can build your own SIP trunk, but it means running an SBC, managing carrier contracts, and owning security and compliance yourself, which is why most teams buy instead.

The process includes many steps: You stand up an SBC, sign a wholesale agreement with a carrier for numbers and minutes, connect the two, then handle encryption, STIR/SHAKEN registration, fraud monitoring, E911 addresses, and failover on your own. A telecom team with an Asterisk or FreePBX background can do it; the question is whether you want to keep doing it.

And if you want to put AI on top of everything, it gets even more complicated. Building the AI layer yourself means four separately billed services and the code to keep them in sync:

• Speech-to-text, to hear the caller
• A language model, to decide what to say
• Text-to-speech, to say it
• The trunk minutes, to carry the call

Developer platforms advertise fees from around $0.05 a minute, but that's the platform line only. The other three bills come on top, and every hop between them adds delay to the conversation. 

If you buy, however, one provider runs the telephony and the AI on one bill, and your PBX stays where it is. Synthflow works this way: it takes calls from an existing 3CX, FreePBX, or Asterisk system and hands them to an AI agent without replacing anything, the way contact center automation has always been layered onto phone systems.

SIP Trunking Is the Layer, AI Is What You Build on It

Whether you're replacing copper lines or adding an AI agent to a trunk you already run, the same three numbers settle most of the conversation with a provider:

1. Your peak concurrent calls, from your current system's logs. That's your channel count.
2. Your PBX and its version, because it decides whether the trunk connects directly or needs a gateway.
3. The calls you'd hand to an agent first, by type and volume. After-hours and appointment confirmations are the usual starting point.

With those in hand, you can choose the right provider in no time, and the AI layer becomes something tangible you can consider. If you'd like a second pair of eyes on the call flow, talk to our team and bring the three numbers with you.

FAQ

What Is the Difference Between IP Trunking and SIP Trunking?

IP trunking is the general term for any phone trunk that runs over an IP network; SIP trunking is IP trunking that uses the SIP protocol to set up calls. Almost every IP trunk sold today is a SIP trunk, so on most quotes the two mean the same thing.

Can You Keep Your Existing Phone Numbers When Switching to SIP Trunking?

Yes. Number portability is a legal right in the US, and the FCC requires single-line ports to finish within one business day. Larger business ports with many numbers typically take two to four weeks, and your old carrier can't refuse the port even if you owe them money.

Can You Set Up a SIP Trunk Yourself?

Yes, if you're willing to run the SBC, the carrier contract, and the security yourself. It's a telecom project, not a settings change, which is why most companies buy the trunk and spend their effort on what runs over it.