We have a Mitel 5000 PBX that was installed by a local ISP close to 2 years ago now. This installation includes a remote user with a Mitel 5320 IP Phone that is on a DSL line from the same service provider and has been working perfectly since the install with a few minor issues during initial setup. Around the first of the month the ISP has an major outage in the Central IL area that included long distance and some DSL lines in our area and ever since that remote 5320 displays a constant "communication error", you can still dial out if you just dial the number you want to call and sound quality is fine it just refuses to load the phone menu and displays the error.
We have moved that 5320 to two other existing DSL lines and the phone works as it should, also if we force the ISP's DSL modem at the troubled location to pickup a different Public IP the phone will work just fine as well but as soon as that DSL Modem gets the Public IP that is assigned to that DSL line the phone goes back to the error state.
I believe we have ruled out the phone itself, the DSL modem, our content filter/firewall and basically everything other than the assigned public IP, I believe that possibly the ISP might have closed a port that should remain open for this system to function properly? Can anyone else help with my troubleshooting here, any ideas or suggestions would be greatly appreciated. Sorry for the long post, I just wanted to give as much detail as I could on what we have done to help isolate the issue. Great forum, I have learned a lot from here already.
