Try a factory reset but backup her important files on an external SD card first

 

Do what @Dannydet said, but it doesn't have to be to an external SD card. You can copy the files anywhere, including a computer, Dropbox, Google Drive, etc. Just make sure they're safely backed up before resetting the phone.

 

I also agree, backing up data and factory reset is good advice. Please let us know if this solved the issues with your phone.

 

I hear you. It is a pain. Well, let's see if we can try something else first. The thing is, it's a transient problem that doesn't appear to follow any pattern. That makes honing in on it very difficult.

You said that it started recently. Were any new apps installed around that time?

 

A factory reset is a bit of a pain.

 

Okay.

You can step through every system setting that's related to phone calls...but I doubt you're going to find your answer there. There is no setting I'm aware of that lets the user choose 'force me to reboot in order to make or receive calls.'

You can also check the settings for any apps that utilize the phone, but...see above.

I'm sorry, but I think a reset is your best bet. It doesn't have to be terrible. Make use of apps' backup features, let Google sync away with its cloud to save everything from contacts to photos, manually clean up anything that's left, and then reset.

Once the phone comes back up and you've entered its Google account credentials, it will automatically repopulate contacts and you'll be able to restore photos and other things you want. Apps that offer their own backups, such as WhatsApp, are easy to restore.

I know this sucks, but unless someone can think of something that's been missed, I think it's your next step.

 

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.

 

Well try to fix the problem itself before falling back on doing a Factory Reset. That should be one of the last options to try, not the first.
As for the backup app you're using, which one is it?

 

Regarding clearing the system cache partition, are you saying you're having a problem booting up into Recovery mode? Or the option to clear the cache partition is missing?

Try double-checking the Settings menu for the Network Settings Reset option:
https://www.hardreset.info/devices/motorola/motorola-moto-z2-force-edition/reset-network-settings/

 

Just to clarify if there's just a syntax issue involved. Safe mode and Recovery mode are two separate, distinct ways to do maintenance on your device, and are accessible in two different ways. Wiping the system cache partition is not accessible in Safe mode. If you are referring to 'Reset' as Recovery mode and there's no option to wipe the system cache, that's a bit odd but apparently something Motorola implemented for this model. (Perhaps attributed to newer versions of Android that are using a newer partition scheme that doesn't involve dedicated system cache partition). I wouldn't worry about it much though, it's unlikely that wiping the system cache partition (whether it's there or not) pertains to this particular issue of having to reboot the phone to do phone calls, especially given it's such happening in such a random way. More of a possibility than a probability. I'd focus more on this issue being tied to the SIM card. Did you try re-seating it or trying it in your phone?