Connecting OpenClaw to a Rotary Phone via SIP and Speech APIs

Hardware and SIP Setup

The setup uses a Benotek rotary phone connected via phone cord to a Grandstream HT801 v2 ATA (Analog Telephone Adapter). The HT801 connects to the router via ethernet and registers as a SIP endpoint with Twilio.

Call Flow: User to OpenClaw

• Pick up the rotary phone and dial the Twilio number
• Twilio answers and plays "Connecting you to OpenClaw"
• Twilio opens a WebSocket media stream to the server via ngrok
• Server connects to Deepgram for speech-to-text and OpenClaw
• Voice audio streams from Twilio to Deepgram in real time
• Deepgram transcribes speech to text
• After 1.5 seconds of silence, the transcript is sent to OpenClaw
• OpenClaw responds with text
• Text is sent to ElevenLabs for text-to-speech conversion
• Audio is converted to mulaw format via ffmpeg
• Mulaw audio streams back through Twilio to the rotary phone speaker

Call Flow: OpenClaw to User

• Hit the /call-me endpoint on the server
• Server tells Twilio to call the SIP endpoint (HT801)
• Rotary phone rings with its mechanical bell
• Pick up, same flow as above

A demonstration video is available showing the system in action.