Voice API Architecture Guide

Voice APIs are event-driven: the platform calls you as the call progresses.

• Understand the call lifecycle events (initiated, ringing, answered, completed) your API emits.Your webhook handler is effectively the call-flow state machine — design it as one.
• Model call control as idempotent webhook handlers that can tolerate retries and out-of-order events.
• Decide between declarative call-flow documents and live API control per event, per use case.
• Handle SIP concepts you can't avoid: early media, DTMF, transfer and re-INVITE semantics.
• Plan number strategy (E.164), caller-ID presentation and per-market caller-ID rules.

• Choose codecs deliberately: G.711 for quality, G.729/Opus where bandwidth is constrained.Budget bandwidth including IP/UDP/RTP overhead, not just the codec bitrate.
• Set a sensible packetization interval (commonly 20 ms) balancing overhead against latency.
• Design for jitter, packet loss and latency: jitter buffers, and realistic MOS expectations.
• Plan DTMF handling (RFC 2833 / SIP INFO / in-band) consistently across the path.
• Decide recording and transcription needs early — they affect media routing and storage.

• Verify webhook signatures and reject unsigned or stale events.An unauthenticated call-control webhook is a takeover risk.
• Respond to webhooks fast and do heavy work asynchronously to avoid blocking the call.
• Make handlers idempotent and deduplicate on event IDs, since retries are expected.
• Correlate call legs with a stable call/session ID across transfer and conference.

• Size concurrent channels from CPS × average call duration, then add peak headroom.Concurrent calls ≈ calls-per-second × average duration in seconds.
• Provision trunk / channel capacity and confirm provider concurrency and CPS limits.
• Scale webhook handlers horizontally and keep them stateless behind a queue.
• Load-test at peak concurrency and confirm graceful degradation, not cliff-edge failure.

• Encrypt signalling (SIP over TLS) and media (SRTP) where offered; restrict by IP allow-list.
• Protect against toll fraud and Wangiri: rate-limit and restrict high-risk premium destinations.
• Design failover across regions/providers so a single outage doesn't drop all calls.
• Log per-call detail records (CDRs) and events, and retain audio only per your compliance rules.
• Instrument quality metrics (MOS, ASR, ACD) and alert on drops per route.