Asterisk - subscribers for incoming calls are periodically not available, what to do?

Question

Asterisk

There is FreePBX 12.0.76.2 as a station.
Phones: Aastra supports 2 or 3 lines (depending on the model).
Internal subscribers: about 50.
In general, everything works well. But periodically (not constantly) and in a chaotic manner, some 1 or 2 devices cease to be available for input (call to an extension), while making outgoing calls (both internal and external) from it is possible.
There is no algorithm for disabling calls to the entrance. The device becomes available after some time (5-10 minutes).
Where to dig, how to be?

Answer 1

Shorten the registration renewal period on the machines. Most likely you have 3600 seconds set. Make 120 sec. Perhaps, for some reason, the PBX "loses" their IP address and port.
In general, as soon as this pops up, immediately look at /var/log/asterisk/messages. Everything will be written there.

Answer 2

At that moment, the problem was fixed - quickly in the SSH console of the PBX
, where XXX is the number of the inaccessible device.
If, when executing the command, you do not see the IP address of your device, it means for sure that the device has fallen off.
If there is an IP address, then do it right away:
make a call to your problem phone and when you receive the end call "CTRL-C".
Logs or see for yourself, or send here.
With tail -f, there should not be a lot of "excess" in theory, since there are only 50 subscribers.