📖 Glossary

SIP trunking for AI calls: what it is and when you need it

Updated 2026-09-19 · 4 min read · By the Convo360 team

In short

SIP trunking is a way of connecting your own existing voice lines, such as a PRI or an ITSP account, directly to an AI calling platform over the internet, instead of buying new numbers through a provider like Exotel or Plivo. It suits businesses with existing high-volume telephony contracts more than businesses just starting out.

Key takeaways

SIP trunking brings your own existing phone lines to the platform, instead of provisioning new numbers through a CPaaS provider.
It generally needs more technical setup — SIP credentials, firewall and codec configuration — than a provider API.
It tends to save money mainly at volume, where an existing telephony contract is already cheaper than pay-as-you-go rates.
For most SMEs starting out, Exotel, Plivo or Twilio's managed APIs are faster to launch and simpler to maintain.
Test call quality and latency with your actual SIP provider before committing a whole campaign to it.

The definition

SIP (Session Initiation Protocol) trunking is a way of carrying voice calls over the internet using a standard protocol, letting a business connect its existing telephone infrastructure, a PRI line, an office EPABX, or an account with an ITSP (Internet Telephony Service Provider), directly to a cloud platform, rather than provisioning brand-new numbers through a telephony API provider.

Where a provider like Exotel or Plivo gives you a ready-made number and a simple API to send and receive calls, a SIP trunk connects a line and infrastructure you or your existing vendor already own.

The underlying protocol itself is the same one that powers most modern office phone systems and internet-based calling generally: SIP is simply the standard way two systems set up, manage and tear down a voice call over an IP network, which is what makes it possible to bridge an existing telecom line into a cloud AI platform in the first place. The word "trunk" itself comes from traditional telephony, where it originally referred to a bulk line carrying many simultaneous calls between two exchanges, and a SIP trunk is the modern, internet-based version of that same idea.

SIP trunk vs provider API: the real difference

	Provider API (Exotel / Plivo / Twilio)	SIP trunk (your own PRI / ITSP)
Setup effort	Low, sign up, get a number, connect keys	Higher, SIP credentials, firewall, codec config
Best for	Most SMEs, quick launch, pay-as-you-go	Businesses with existing high-volume lines
Ongoing cost model	Per-minute, billed by the provider	Whatever your existing PRI/ITSP contract charges
Technical ownership	Provider manages the telephony infrastructure	You or your telecom vendor manage the trunk

When SIP trunking actually saves money

SIP trunking tends to make financial sense for businesses that already run a call centre with an existing PRI line or a bulk ITSP contract, where the per-minute cost is already lower than a pay-as-you-go telephony API, and the volume is high enough that the extra setup effort pays for itself. A business making a few hundred AI calls a month rarely sees the same benefit. The technical overhead outweighs the savings at that scale.

It is worth genuinely comparing your existing telephony contract's per-minute rate against a provider API's published rate before assuming SIP is cheaper; at low to moderate volumes, it frequently is not.

The other scenario where SIP trunking earns its setup cost is regulatory or operational: some larger organisations are required, or simply prefer, to keep voice traffic routed through a specific telecom vendor they already have a compliance relationship with, rather than adding a new telephony API provider into the mix purely for AI calling.

Setup pointers

Get SIP credentials from your ITSP or PBX vendor — typically a SIP URI, username, password and allowed IP addresses.
Configure the trunk in your platform's phone number settings using those credentials.
Check firewall and NAT rules: SIP traffic needs specific ports open, and misconfigured firewalls are the most common cause of calls connecting but dropping audio.
Confirm codec compatibility between your ITSP and the platform, since a mismatch can cause poor audio quality even when the call itself connects fine.
Test with a handful of real calls before routing a full campaign or an inbound line through the new trunk.

Common mistakes

Assuming SIP is always cheaper: at low volumes, the setup and maintenance effort usually costs more than a simple provider API would have.
Skipping firewall configuration: this is the single most common reason a SIP trunk connects but calls have no audio or drop immediately.
Not testing latency end to end: an ITSP with poor routing to your AI platform's servers can add noticeable delay, which matters far more for a live AI conversation than for a normal human call.
Switching an entire live campaign over at once: test the trunk on a small slice of traffic first, in case an issue only shows up under real call volume.
Forgetting to plan for redundancy: a provider API typically handles failover for you; a self-managed SIP trunk may need its own backup route if the primary connection goes down.

How it shows up in Convo360

Under Phone Numbers, Convo360 supports connecting a number from Exotel, Plivo, Twilio, or your own SIP trunk. Whichever you choose, it remains fully BYOK — you pay your telephony provider or existing trunk contract directly at its own rate, with no per-minute markup added by the platform. Most businesses starting out do better with a managed provider API for speed of setup; SIP trunking is worth evaluating once call volume and an existing contract make the extra configuration clearly worthwhile. See choosing a telephony provider in India for a fuller comparison. Whichever connection method is used, the same voice agent, system prompt and Flow Builder logic runs unchanged, and switching from a provider API number to a SIP trunk later is a change to how calls arrive, not a rebuild of the agent itself.

Example conversation

We already have a 30-line PRI for our call centre. Can we use that instead of buying new numbers?

Yes, that's exactly the case where a SIP trunk makes sense. We'll connect your existing PRI through your ITSP, and your AI agent can start taking calls on those same lines.

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Frequently asked questions

Is SIP trunking cheaper than using Exotel, Plivo or Twilio?

Only usually at higher call volumes with an existing telephony contract already cheaper than pay-as-you-go rates. For most small and growing businesses, a provider API is simpler and not meaningfully more expensive.

Do I need technical staff to set up a SIP trunk?

Generally yes, or at least support from your ITSP or telecom vendor, since firewall, codec and SIP credential configuration are more involved than a provider API's plug-and-play setup.

Can an AI voice agent use a SIP trunk for both inbound and outbound calls?

Yes, once configured correctly, a SIP trunk can carry both inbound calls to your AI receptionist and outbound calls for campaigns, the same as a provider API number can.

What is the most common problem with a new SIP trunk setup?

Calls connecting but having no audio or dropping shortly after, almost always caused by firewall or NAT configuration blocking the required SIP and media ports.

Can I switch from a provider API number to a SIP trunk later?

Yes, this is a common path as call volume grows — start with a provider API for speed, and move higher-volume lines to a SIP trunk once the setup effort is clearly justified by the savings.