The customer is unable to make calls using their UK SIM card while in Belgium.

Can you try putting the SIM card in the second slot by any chance? You have to use only one SIM slot. If you're outside the country, just to make sure that it will work. So that's with only one SIM slot. Leave the other one empty and try the second one, please.

If you have a friend that has the same exact SIM card, and he's right with you, you can ask him that his SIM card, put it in your phone and try to make a call. If his SIM card that you're telling me that it's working with another Fig phone, it's working, and it works when you make a call, that means it's not the phone. Now, if it's working on his phone, and it's not working on your phone, then we can say that it's the phone, and at least we can make a reset or something like that.

Can you try with local Belgium SIM card to see if it will work?

As long as the phone is making calls, you're receiving text. So that means that it's working. Unfortunately, I'm afraid that I can't provide you an immediate solution with the things that you're having.