You have probably seen OpenAI Realtime API demos — ultra-low-latency, natural voice conversations with GPT-4o. Impressive in the browser. But what about a real phone call? Your users are not always at a computer. They call. Turning that browser demo into an actual phone number is where most developers get stuck. This post walks through the architecture and code to put GPT-4o voice on a real phone line — without writing a single line of RTP or SIP code. The Core Problem OpenAI Realtime API speaks WebSocket. Phone networks speak RTP (Real-time Transport Protocol) — a completely different audio streaming format that requires: SIP signaling to handle call setup and teardown RTP stream processing for audio delivery Audio codec transcoding (G.711 ↔ PCM16) Network jitter buffering and packet loss handling Most developers do not want to build this. They should not have to. VoIPBin acts as the translation layer.…