Hello Vapi Support, we are running into a major issue with the "First Message" audio playback: Symptoms: • The First Message plays perfectly in the dashboard, but is distorted or wrong-pitched on the phone (via Twilio) with the second use case it’s exact the other way arround. • The behavior is not consistent: Sometimes it works in the dashboard and sometimes on the phone. Test Cases: 1. output_vapi_smallest.wav URL: https://cdn.jsdelivr.net/gh/bdm2m/first-messages/output_vapi_smallest.wav o Format: WAV, mono, 8 kHz, µ-law (pcm_mulaw), very small file o Result: Plays fine via phone/Twilio, but fails in the dashboard wrong pitched or slowed down 2. output_vapi_recomendation.wav URL: https://cdn.jsdelivr.net/gh/bdm2m/first-messages/output_vapi_recomendation.wav o Format: WAV, mono, 44.1 kHz (or 48 kHz), PCM Linear (pcm_s16le), typical web-optimized o Result: Works in the dashboard, but fails in the via phone/Twilio wrong pitched or slowed down is distorted or silent via phone/Twilio. Additional info: • Source files are generated via ElevenLabs (MP3), then converted with ffmpeg, e.g.: ffmpeg -i input.mp3 -ar 8000 -ac 1 -c:a pcm_mulaw output_vapi_smallest.wav ffmpeg -i input.mp3 -ar 44100 -ac 1 -c:a pcm_s16le output_vapi_recomendation.wav • Vapi allows only one firstMessage file, which must work for both use-cases. Key Question: Which exact audio format (codec, sample rate, bit depth, mono/stereo) is 100% compatible for First Message, so it works cleanly both in the dashboard and in phone calls (Twilio)? • Do you have an official “golden standard” or recommendation? • Is it a known issue that PCM-mulaw (8kHz) fails in browser/dashboard? • Can your backend deliver different formats depending on context (dashboard vs phone)? • Or do you have a best-practice workflow for universal compatibility? Thank you very much for your help!

Hey, could you send me a screen recording of the issue you're facing? That would help me understand better. Also, please provide a call ID where you experienced this issue so we can review the logs.