Bring your own SIP trunk: AI voice agents on the carrier you already use

Wixzel Voice is a voice AI API for building AI agents that place and answer real phone calls over your own SIP trunk, or talk to people in your web and mobile apps. One API key and one prepaid balance cover every voice engine, billed per second of actual usage.

Wixzel Voice is the voice AI API for building AI phone agents, and it stops at the voice. The phone line is yours. That is a deliberate trade: it means one more step before your first call, and it means Wixzel Voice is never between you and your telephony provider. There is no markup, no resale margin, and no reason for Wixzel Voice to care which carrier you pick.

• Your numbers stay yours. Wixzel Voice does not sell or port numbers, so leaving costs you nothing but a setting at your carrier.
• Your carrier rates stay yours. The carrier bills you directly for the line. Wixzel Voice charges only for the voice engine, billed per second of actual usage from a prepaid balance.
• Your compliance stays with your carrier. Caller ID provisioning, KYC and destination registration are between you and the carrier you already trust with them.

A Wixzel Voice trunk has two halves, and they live in different parts of your carrier's console. Both addresses are returned on every trunk, so read them from the API rather than trusting a page to stay current.

Every carrier below has its own page with what to buy, what to allowlist, where inbound calls go, the trunk request that fits it, and the gotchas that are specific to it, with a link to the full console steps in the docs. Each carrier names the two settings differently; the setting is the same.

Two more settings decide whether a correct-looking trunk works. Number format: Twilio, Telnyx and Vobiz expect the leading +; many wholesale carriers cannot parse it and time out with 408, which send_plus: false fixes. Transport: UDP is the default almost everywhere, but Exotel does not take UDP, so an Exotel trunk runs over TCP, and a mismatch looks like the carrier ignoring you.

A call the API accepted and the carrier then refused shows status: "failed" (or busy for cause 17 and no-answer for cause 19) on GET /v1/calls/{id}, with failure_code (the Q.850 cause from the carrier, the stable thing to branch on) and failure_reason (what to do, in plain language). Its cost_micros is null: a call that never connected costs nothing, and the credit reserved for it is released.

Read your carrier's own call log first. If the call is not listed, it never reached the carrier: the allowlist, the trunk host or a firewall is wrong. If it is listed with zero answered and zero cost, the carrier accepted the INVITE and then cleared it, which is a carrier permission problem far more often than a configuration one: the caller ID you present must be authorised on the account, KYC and any destination registration must be complete where the market requires them, and the destination range must be enabled, not just funded. These are the causes the API explains:

These are refused by POST /v1/calls before anything is dialled, and cost nothing:

GET /v1/sip-trunks/{id}/status shows whether the trunk is reachable right now and lists its last few carrier rejections; POST /v1/sip-trunks/{id}/test probes it on demand, and GET /v1/sip-trunks/{id}/logs returns the SIP log.

An inbound number that never rings is not a failure you can look up: if the carrier has no destination set, the call never leaves the carrier, so there is no failed call to inspect. Check the inbound destination, that the number is attached to the trunk on the carrier side, and that it has an inbound_agent_id. If calls connect but nobody hears anything, the media is not getting through: a firewall in the path has to allow the RTP range, not just the signalling port.