Why does your site insist my mobile tel no is "Invalid"?

I am living in Italy but I have a British smartphone with a British number, no matter what format I type my number the site insists it is an invalid number. Why can't you use a valid mobile number unless it contains the international dialling prefix of the country you are in at the time? This excludes foreign workers and ex-pats from syncing firefox between computer and smartphone if that phone number is not from the country you are in....