is there any callback event in ACS if user not picking the call and call get transfers to voice mail

You may consider below:

1.    Start mixed audio-video recording
o    Call startRecording with audioVideo format and your storage locator.
o    Capture recordingId and track status; stop when the call ends or by policy.
2.    Start server-side audio bot
o    Join the call using the server SDK with media access enabled.
o    Subscribe to remote audio streams; attach event handlers for participant add/remove.
o    Write PCM frames to separate files per participant.
o    Embed timestamps (RTP timestamps or app-level monotonic time) for alignment.
3.    Handle edges and state
o    On participant mute/unmute: write silence markers or zero samples to keep alignment.
o    On network jitter: buffer slightly (e.g., 100–250 ms) to smooth out write cadence.
o    On rejoin: start a new segment file or append with an offset note.