Assuming you have other working TW phones, have the user default the phone... The procedure is slightly different depending on software version of the phone, but essentially, plug in the phone well holding the Volume Down button and continue holding it until a menu appears, navigate to Tools, enter that menu and look for Restore Defaults or Reset Defaults. Then the phone will reboot, hold down the 7 button to enter Teleworker mode and reconfigure your TW IP address, save and reboot.
If this doesn't fix your issue it is likely the firmware has been corrupted, and I would replace the set.
Assert errors are usually indications of set related problems...
