I have got FreePBX setup @home and for some reasons my voicemail is not working properly. It basically drops the connection after 30 seconds while the person on the phone is waiting on Music on Hold.
It turns out the FreePBX detects there was no audio/RTP activity within 30 seconds (configurable) and drops the connection.
To change this: on your FreePBX, navigate to: Settings > Asterisk SIP Settings > Media and RTP Settings
Change the rtptimeout from 30 to 300, change rtpkeepalive from 0 to 30
