Phones Lose Registration

[shmuel@birns.net]

We have about 1000 Yealink phones out there connected to our hosted PBX system. Every so often a customer will call us and say that their phone is not working. I log into their phone and the accounts page either says Register-Failed or Disabled. Sometimes I can hit confirm and the phone then registers, sometimes I disable the account and re-enable and it registers, and sometimes none of that works. When those don't work, I change the Local SIP Port under Accounts>Advanced from 5060 to some random port and then it registers.

I need help in trying to figure out why they keep going down.

This is with T2X phones and T4X phones.

All of them are on firmware version 73.X for the T4X phones and 72.X for the T2X phones.