Had a strange issue today where after our internet connection had been off for some time our VOIP phones unregistered and would not register on our PBX.
This was very unexpected as our PBX runs over ISDN and not TCP/IP, the phones connect to our PBX on our internal LAN so am struggling to figure out why the lack of internet would effect our phones in this way.
We use OSS Endpoint manager to configure the phones on the PBX, on the network they all get their IP addresses from an internal DHCP server using our internal firewall as the default gateway, which was still up and running, I was also able to Telnet onto the phones to check IP address, DNS etc. The reason we set it up in this way was so that if the internet went down we would still be able to speak to our customers on the good old fashioned ISDN lines. The phones do use a URL to obtain a logo for the display (all Cisco 7940’s) but other than they should not need the internet to work.
Can anyone think of a reason why this our system behaves in this way so that I can fix it?