What is a SIP trunk?
The virtual phone line that connects software to the public phone network, so a voice agent can make and receive real calls.
SIP trunk — A SIP trunk is a virtual phone line that connects a phone system to the public telephone network over the internet using the Session Initiation Protocol. It is how a voice agent or softphone gets real phone numbers and carrier connectivity.
Why it matters
A voice agent or a softphone does not connect directly to the public phone network. It connects to a SIP trunk, a virtual line delivered over the internet using the Session Initiation Protocol, and the trunk provider is the one actually carrying the call to and from real phone numbers on the carrier network. Without a trunk, an agent can process audio but has no way to actually place or receive a call from a phone number.
SIP trunking is also where a chunk of the real cost of running a voice agent lives, separate from whatever the AI platform charges for its own usage. Per-minute telephony rates, number rental, and sometimes per-channel capacity limits are billed by the trunk provider, and they vary by country and by whether calls are inbound or outbound. A quoted per-minute price for a voice agent platform frequently does not include this leg at all.
Where a SIP trunk sits
- 1
A phone number is provisioned
A real, dialable number is assigned, usually by the trunk provider or a carrier.
- 2
The SIP trunk connects it to software
The trunk carries the call over the internet to the voice agent or phone system.
- 3
The voice agent handles the audio
Once connected, the agent's own speech and language systems take over the conversation.
- 4
Billing runs on two tracks
The trunk provider bills for telephony minutes and numbers; the AI platform bills separately for its own usage.
The mistake to watch for
Questions
- How is a SIP trunk different from a VoIP provider?
- A SIP trunk is the specific connection type, the virtual line itself, using the Session Initiation Protocol. A VoIP provider is the company selling that connection, along with phone numbers and often other services layered on top. Most VoIP providers deliver their service as a SIP trunk under the hood.
- Does every voice agent platform need its own SIP trunk?
- It needs a SIP trunk from somewhere, but not necessarily its own. Some platforms include telephony as part of their price, others expect the customer to bring a trunk from a separate provider like Twilio, and the difference materially changes the true per-minute cost of running the agent.
- Can one SIP trunk handle multiple simultaneous calls?
- Yes, within the channel capacity the trunk provider sells. Trunks are typically priced or capped by the number of simultaneous calls they can carry, which is a separate limit from the per-minute rate and matters for any business running more than one call at a time.