I have run into this a few times and google is not finding me any answers today.
I have noticed with freepbx 12 and 13 running on local hardware with a PRI, if the default gateway goes down
for any reason my phones stop talking with the pbx.
I believe its just new sessions that do not work and that existing calls continue to work but I would have to test to be sure.
Both the phones and PBX are on the same lan subnet.
Any one else run into this or have a fix?