I have had a Freepbx system running for a while now with Sangoma S505 phones connected to it working just fine. Recently, some of the phones started to give an XML error when pressing the VM button on the phone itself, with or without voicemails waiting. The other App buttons on the screen work still. It started with 1 phone a few weeks ago, and seems that more phones are being added to the list as time goes on.
I have verified that the system is up to date, updated the phone apps, updated the firmware on the phones, factory reset the phones, verified they are on the same internal network with no firewall, and checked to make sure phone apps were enabled for the extensions.
I am not sure where to go from here.