Hello, I recently purchased a PAYG UK Sim from Three, to use in my Samsung Galaxy Captivate (American AT&T Model). This phone is unlocked, and I have been using it in Ireland with O2 for the past year and a half with no issues. However with the Three sim, while I'm able to send/receive texts, and use 3G, when I make or receive phone calls the other party is not able to hear me, and I can't hear them. Occasionally the phone call with work, however 99.9% of the time it just wont work. I have been in to the Three stores and they said my phone isn't compatible.... but then how come text and 3G works, and why on occasion does it work when I call people? Any help would be reaallly appreciated.