Separate from agent sip-client.ts — different lifecycle (on-demand per
barge session, not persistent). Auto-answers incoming Ozonetel calls.
DTMF mode switching: 4=listen, 5=whisper, 6=barge. Event-driven with
registered/callConnected/callEnded events. Audio via hidden <audio> element.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>