initiate facetime calls from Italy to the USA

I have an iPhone 4s with an up to date OS. My sim is a US based phone number. I am in Italy and can receive facetime calls from the USA fine. When I attempt to initiate a facetime call from Italy to the USA facetime always states that the caller is unavailable. Even when testing this, the USA based user can always call me, even when just seconds before or after, I cannot initiate a call to them. Any ideas?