Please help getting Ebay phone to work

My daughter dunked her Motorola T720 and I got her the same one (used) on Ebay. We called Cingular and gave them the ESN numbers etc. When we turn the unit on it powers up and goes to "searching for service" for a while and then goes to the main screen showing a tower with no bars. Every call fails. We took it to a store and the guy checked out the sim card and said it was OK. We also tried driving right up under a tower (per their suggestion) and powering on to try to get just one bar. Still nothing. When we power up it says Cingular on the back of the phone so it seems to be seeing the system and when it powers up it does not tell us that it cant get service it just starts with a tower and no bars.

Can anybody help? thanx

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You may need a new sim card.

Thanks. We tried that. I also called a place called Universal Cellular repair in Fla. They said this is often caused by an internal break where the antenna is connected. Unfortunately, they recently said that the repair was not that simple and they would have to replace the MB.
dan