Not a certainty, but that issues typically happens on devices where a clone file is used across a generational gap in firmware.
The 59xx/58xx/6655/722x/78xx/79xx/3655 are all Connectkey products. If you take a clone file from a 58xx and put it on a 59xx it will break fax.
It will also cause major issues if you use a clone on a firmware that starts with 072 (Connectkey 1.5), from a machine that has one beginning with 071 (This is Connectkey 1.0)
If that is the case, the only way to fix it is with an Altboot no data backup no clone, which means the tech wipes the device, brings it to the latest release, and you set it back up manually.
We have now moved on to an 073 release (Oddly, we are not calling it Connectkey 2.0) that will not allow clone files if they come from other devices or incompatible firmware.