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?
This happens because you loose DNS. Asterisk chan_sip requires DNS if your using a FQDN on anything like a trunk setup. Switch to a IP address and it should work fine.
The PBX and phones are all IP based, the systems have a PRI and card in them so trunks are local.
I have 12 sites to upgrade firewalls for and will get some data back as to which ones are experiencing this
with versions and network settings.
This is something I have recreated several times while onsite and working the networks.