Glossary
SIP trunk
A SIP trunk is a telephone line over the internet that connects a phone system or an application to the public telephone network using the SIP protocol.
SIP stands for Session Initiation Protocol. It sets calls up and tears them down; the speech itself is carried as a data stream. A trunk bundles several simultaneous calls and one or more blocks of phone numbers. It has replaced the ISDN lines that phone systems used to be connected to.
For AI telephony, a SIP trunk is the direct way to connect an existing phone system to an assistant: the phone system or the provider sends selected calls to the assistant via SIP, and the assistant can hand them back into the phone system. Compared with simple call forwarding, more information is preserved, such as the caller’s number. In return, the setup is more technical and usually needs support from the phone provider or the IT department.
In practice
Setup usually happens in the phone system or in the telephone provider’s portal. There you define which calls go to the assistant: a particular extension, all calls outside business hours or the overflow of a queue. You need to agree on credentials or permitted IP addresses, the format of the phone numbers and the audio codecs both sides support. Plan a test with incoming calls, with a handover back into the phone system and with several calls at once before real callers arrive by this route.
An example
An IT service provider runs a cloud phone system with a queue for its hotline. The system sends every call that waits longer than half a minute to the phone assistant via SIP. The assistant sees the caller’s number, records the fault report and hands urgent cases back to the extension of the on-call technician. For customers it remains the same hotline number, and all calls still appear in the call list of the phone system.
A common misunderstanding
SIP trunk and call forwarding are frequently treated as the same thing, because both bring a call to the assistant in the end. With forwarding, your provider sets up a second call to another number. With a trunk, phone system and assistant are connected directly. Another misconception is that a SIP trunk is only for large companies. Cloud phone systems for small businesses offer it too. Whether the effort pays off depends on whether you need the additional information and the handover back into the system.
At Call0
An existing number can be connected to Call0 via SIP, via Twilio or with call forwarding.
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As of: 12 October 2026