How a Voice AI Call Transfer Shipped Broken and Logged Success for Every Failed Call

A development team building AI voice agents discovered their first call-transfer implementation never delivered a single call to human agents, despite internal logs consistently recording the outcome as successful. The flawed approach used a SIP REFER method, which received a 200 OK acknowledgment from carrier Telnyx but silently dropped callers without ever ringing the destination. Investigation revealed REFER-based transfers only work on inbound calls to a registered number with a specific Diversion header — a configuration incompatible with the team's outbound call setup and unsupported by LiveKit's transfer API. The team rebuilt the feature using a bridged second call dialled directly into the same conversation, with a 45-second answer timeout and explicit agent silencing to prevent the AI from continuing to speak or transcribe after handover. The new system trades lower per-minute costs for reliability, and correctly handles edge cases such as unanswered transfers, non-phone channels, and unauthorized destination numbers.