Motorola Slvr Emergency Only Mode

I have a Motorola Slvr that my daughter had purchased from ebay to use
on the Cingular network. The phone works good most of the time but at
various times will change to "Emergency Only" mode and will not receive
calls nor can you make any calls except to 911. The phone returns to normal
operation sometimes in a few minutes, other times it may stay in the Emergency mode for an hour or more.

I had it checked today at a Cingular store and they attempted to do a master
reset but were unable because someone prior to us had changed the 4 digit
code, assuming he meant the security code.

Does anyone have any ideas what I might be able to do to get this phone
working properly?

Thanks,

Jim