Your AI agent needs a phone line

AI voice agents connect to real phone numbers through a SIP trunk — a carrier-provided voice channel that bridges your AI platform to the Public Switched Telephone Network (PSTN). The architecture has three layers: a SIP trunk carrier (such as IDT Express) provides the phone numbers and PSTN routing; an AI calling platform (Pipecat, Vapi, Retell, or a custom stack) terminates the SIP call and receives the RTP audio stream; and an AI pipeline (STT → LLM → TTS) processes the audio and generates a response. The synthesised audio is sent back through the SIP trunk to the caller. Every AI voice agent that makes or receives phone calls uses this architecture, regardless of which platform or framework it runs on.

G.711 (G.711u/PCMU for North America or G.711a/PCMA for international routes) is the correct codec for AI voice agents. G.711 is an uncompressed, low-latency codec that requires no transcoding at the carrier level, eliminating the 5–20ms transcoding delay that codecs like G.729 introduce. Since AI voice agents already run compute-intensive STT and TTS models, every millisecond saved at the media layer matters. IDT Express passes G.711 natively — no transcoding, no additional latency in the media path. Configure your SIP trunk and AI platform to negotiate G.711 as the preferred codec and disable G.729 in your codec priority list.

BYOC (Bring Your Own Carrier) is a feature offered by AI calling platforms — such as Vapi, Retell, Bland, and Twilio — that lets you connect your own SIP trunk provider instead of using the platform's default carrier. By default, these platforms route your calls through their own carrier at a retail margin. With BYOC, you configure IDT Express as your SIP trunk provider and your calls route through IDT Express at wholesale rates — typically 60–80% cheaper per minute than platform-bundled carrier pricing. Setup requires providing your IDT Express SIP credentials to the platform and configuring a SIP trunk in the IDT Express portal pointing to the platform's SIP endpoint.

IDT Express SIP trunking has no hard concurrent call limit for AI voice deployments. Channels scale elastically — from 1 to 10,000+ concurrent calls without pre-provisioning or capacity negotiation. This is critical for outbound AI campaign deployments where call volume ramps sharply. Unlike traditional SIP trunking contracts that require you to pre-purchase a fixed number of channels, IDT Express charges per-second on actual usage, so your cost scales linearly with actual call volume rather than reserved capacity.

IDT Express delivers sub-50ms one-way media latency from carrier ingress to your AI platform endpoint when your platform is co-located near an IDT Express Point of Presence. IDT Express operates PoPs in North America, Europe, Asia-Pacific, and Latin America. For AI voice, the practical target is to keep carrier media latency under 50ms one-way so the total round-trip budget — including STT inference, LLM response, and TTS synthesis — stays under 800ms end-to-end, the threshold for a natural conversational experience. Minimising carrier latency is the single most controllable variable in the AI voice latency stack.

To give your AI voice agent an inbound phone number, provision a DID (Direct Inward Dialing) number through IDT Express. DIDs are available instantly across 160+ countries — US local numbers, UK geographic numbers, Australian numbers, and more. Once provisioned, configure the DID to route inbound calls to your AI platform's SIP endpoint. When a caller dials your DID, IDT Express delivers the call as a SIP INVITE to your AI platform, triggering your agent pipeline. For outbound-only AI agents — automated dialers, appointment reminders — you do not need an inbound DID, but you do need a valid outbound CLI, which IDT Express handles as part of standard SIP trunk configuration.

Yes. IDT Express SIP trunking is compatible with Pipecat, Vapi, Retell, Bland, LiveKit, Amazon Connect, and any platform or custom stack that supports standard SIP trunking. For platforms with BYOC support — including Vapi, Retell, and Bland — configure IDT Express as your carrier in the platform's BYOC settings. For Pipecat and other self-hosted AI agent frameworks, configure your SIP trunk credentials directly in your transport or SBC layer. IDT Express provides configuration guides for all major platforms and offers technical support for integration.