I'm in France and I wanted to use Firefox Sync for the first time. I had to insert the phone number in order to add a second device and the system had already filled the form with +33 that is the international prefix for french numbers. The problem is that I don't have a french phone number and I wanted to use another prefix but it is impossible. I tried using 00 instead of plus or directly without the prefix but I always get as answer that "the phone number is invalid". Could someone help me to understand which is the rule used to validate the phone number? Thank you in advance.