SIP trunk
A SIP trunk is an IP-based connection that links a business phone system or voice application to a telephony provider for inbound and outbound call routing. It replaces the role of a bundle of physical phone lines with a logical connection that can support multiple call sessions.
How a SIP trunk works
The trunk uses SIP signaling to establish and manage calls between the customer endpoint and the provider. Audio is carried separately as a media stream. The provider can connect those calls to phone numbers and the PSTN, while the customer side may be a PBX, contact-center platform, session border controller, or AI voice application.
Configuration commonly identifies where each side should send calls and how the connection is authenticated. It can also define accepted number formats, codecs, source addresses, encryption options, concurrent-call capacity, and failover behavior. Some trunks authenticate with credentials; others trust traffic from approved network addresses. The exact requirements come from the two systems being connected.
A SIP trunk is not the same thing as a phone number. A DID number identifies a reachable destination, while the trunk provides a route over which calls to or from that number can travel. One trunk may serve many numbers, and number ownership or verification may be handled separately from the signaling connection.
Why it matters for AI phone calls
A SIP trunk can let a business keep an existing telephony relationship while routing calls to an AI agent. It also gives the implementation team control over number mapping, inbound destinations, outbound presentation, transfer paths, and capacity. That flexibility comes with operational responsibility: both signaling and media must be tested, and misconfigured routing can produce rejected calls, one-way audio, or incorrect caller identity.
Production testing should cover inbound and outbound calls, simultaneous traffic, codec negotiation, number formatting, transfers, unavailable destinations, and recovery after a network interruption. A successful single call is only one part of trunk validation.
In practice on ThunderPhone
ThunderPhone supports manual SIP configuration for any SIP trunk, as well as supported direct VoIP connections. Verified imported production numbers support inbound and outbound calling. Demo U.S. numbers are inbound-only and intended for testing rather than production.