Before going through the project of a Factory Reset, it might be more prudent to address the problem more specifically. Since the issue appears to be tied to phone calling:
-- Try removing the SIM card and replacing it. If the card loosened in its slot, just re-seating it might help.
-- If you try putting the SIM in your phone, does it still have that same problem? If yes, it might be because the card itself has failed, contact her carrier to get a new on.
-- Try restarting her phone into its Safe Mode:
https://www.hardreset.info/devices/motorola/motorola-moto-z2-force-edition/safe-mode/
While in Safe Mode only the base Android operating system gets loaded into memory at startup. No third-party apps or services get loaded. Try a do a phone call test in Safe Mode -- note that most apps take longer to start as nothing except the OS is running so every app/service has get started up from scratch.
If phone calling isn't a problem in Safe Mode that might indicate some app that's been installed is causing the problem. You now have to go through the process of trial and error to determine just which one. It's a hassle but at least indicates what to look for.
-- If you look in her Settings menu, there will be a 'Network settings reset' option (Maybe the Settings >> Wireless & networks >> More menu, its location varies depending on the manufacturer and version of Android). This option will wipe all network-related settings, saved WiFi networks, Bluetooth pairings, etc. too, not just the cellular settings so it's pretty thorough. Essentially a Factory Reset but limited to just network issues.
Addendum
This might not apply depending on which version of Android is on her phone but you might also want to just clear the system cache partition. The system cache is completely separate from things like app caches, and its partition is completely isolated from the user data partition so there's no chance it will delete any of her files, documents, and such. It's unlikely the system cache has any bearing on this phone call issue but it doesn't hurt anything to do it in the event is does (cache files are always just temporary working files, they don't retain data). Anyway, you need to reboot her phone into its Recovery Mode to wipe the system cache:
https://www.hardreset.info/devices/motorola/motorola-moto-z2-force-edition/recovery-mode/
It's a text only interface so use the indicated buttons to navigate through the various options and select the 'Wipe cache partition' option. Should only take a few seconds.