problem on my w810i

just this week or two, my w810i start to have problem.
when everything seems fine, got the title "cingular" on the screen, the 5 bars are full. But when I try to make a call, it drops 2 seconds after i pressed the call button (this is NOT drop call like disconnect in the middle of a call), and the cell phone gives me several beeps (no idea what it means). In this situation, I can't even receive calls.
In the worse time, it even said "no network coverage", or got a small "no call" icon on the top of the screen, or got both, but the 5 bars are STILL full!!!
I don't know if it is cingular's problem for my phone's problem. This issue just happened about a week.
This is really confusing and troublesome. I mean, what is good of a cell phone if it cannot make and receive calls?? Reset the phone or even take the battery out won't help. A cingular guy last night said it is the IMEI number problem, and say it's fixed, but the problem still here, unfortunately.

any help?

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Just return it to save you the hassle with trying to tinker with it any further. Call ATT/Cingular and just tell the rep. Even though your 30-day trial has ended for returning a phone, there is still the 1-year warranty, and ATT will honor it.

ends up it is the sim card's problem
the guy on phone is willing to give me a new sim card, for free, with overnight shipping by UPS (from TX to NY) without costing me a dime!!!
one thing funny though, the title now changed from cingular to AT&T.

Yup, new activations will now be recognized as ATT instead of Cingular. Existing Cingular customers can also change the operator logo on their phones by either calling up CS or by going to a COR store.

If your SIM is taking a while to arrive, you can just go to a COR store and pick one up there.

nah, I don't know if they will charge me
anyway, everything is fine now, hopefully this problem won't happen again.