GU 87 problem, please help!

Hi, I have an unlock Gu87 from AT&T... Everything was working fine till a few days ago, when suddently the phone doesn't accept any incoming or outgoing calls... It will work randomly, ie sometimes it'll work sometimes it won't. I haven't drop the phone or anything, and when I switch sim cards it's the samething... If you know whats going on, please let me know ASAP. Thanks

just get a replacement.