Setup checklist
1.PLIVO_AUTH_ID — Plivo account SID
2.PLIVO_AUTH_TOKEN — Plivo auth token
3.PLIVO_CALLER_ID — outbound sender (optional, auto-discovers first owned number)
4.ELEVENLABS_API_KEY — for natural voice (optional, falls back to Plivo TTS)
5.VOICE_BASE_URL=https://voice.ratesight.com — public webhook URL
6.Configure Plivo inbound SMS URL → https://voice.ratesight.com/sms/inbound