Building a Voice AI Phone Agent: Why Microphone Timing Is the Hard Part

Developers building real-time voice agents on Twilio Media Streams often encounter a critical feedback loop where the agent's own speech is picked up by its speech-to-text system, causing the bot to respond to itself. The root cause is premature microphone reopening — triggered when the audio stream ends on the server side, not when the caller actually hears the last word. Twilio's mark frame feature offers a reliable fix by signaling the exact moment playback reaches the caller's ear, allowing the microphone gate to open only then. Beyond that single condition, a production-ready system requires four additional checks — including an empty speech queue, no active TTS stream, and a non-generating LLM — to prevent race conditions and cut-off sentences. The author, running this system on a live German phone line, notes that these bugs rarely surface in local testing and only appear on real mobile networks with higher latency.