All lines are busy

Good afternoon. There are several FreePBX stations in the network, the trunks are configured, everything works, but from time to time you cannot call one of the stations (error: all lines are busy) or make a call from it. Rebooting solves the problem for a few days. Where to look for a problem tell me ??

The logs - without them it is all guesswork.

The router - my guess is that a NAT or firewall rules is timing out and not getting re-instated.

спасибо. проверю

You can also try to change the transport from UDP to TCP, that solves many problems sometimes.