A SIP trunk is a virtual phone line over the Internet. SIP (Session Initiation Protocol) is the VoIP standard used to route phone numbers to endpoints — such as an AI phone assistant. Instead of ISDN or analogue lines, packets travel over IP.
During onboarding there are two paths: port existing numbers to the new vendor (typically a 4–6 week wait depending on the losing carrier), or provision a fresh SIP number instantly. Key parameters: concurrent channels, codec support (G.711 / Opus), country codes.
Operational pitfalls: emergency-call routing (in Germany 110/112 must be guaranteed — not trivial on pure SIP), fax (use T.38, not G.711), and resilience via multiple trunks or providers.