iPhone 5 dropping calls + power button problem

Hello everyone!

I'm writing outside of USA. I bought from someone a new iPhone 5 with AT&T sim-card in it on December 2012. I unlocked it via some guys from eBay. They unlocked it for 4$ within 24 hours, I don't know how they do it. Then I activated, everything was going well but after February 2013 it started to drop calls, or when someone calls me we can talk about 4-10 seconds and caller or me can't hear each other and suddenly it is dropping calls. Later I faced another problem, the power button is not working properly, I have to push left side of the button hardly.

I told with Apple support, they said that I have to send to any friends in USA, they'll fix it because it has a warranty for one year.

But, the biggest problem is, I don't really know about AT&T contract. What should I do? I don't know maybe it was reported as stolen before leaving the USA. But when I provided serial number to Apple Support, they didn't told me anything. I'm planning to send back to USA, what problems I might face with? And can you please guys tell me any advises?

Thanks a lot in advance guy! Will much appreciate your help.

Best Regards.

Isken.