Reasons for dropping a call?

Question

Asterisk

What is the most common reason for disconnected calls?
The situation is this: the call arrives at the goip gateway > Freepbx extension number > follow me forwarding through the zadarma trunk to a mobile number. It works, the conversation lasts about 1 minute, then the subscriber who received the incoming call is dropped from the line, he receives a call again and returns him to the line. And the subscriber who called and remains all the time on the line. Everything worked for several months without problems and for no apparent reason broke. Changed only qualify to 4000 instead of yes (2000). returned back - not that. Reinvite settings - No, RTP timeout - 30, RTP hold - 300.
Where to dig?

Answer 1

via zadarma trunk

Maybe this is the reason?