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Did they jump the signal or obtain her electronic SSN and program another phone with the SSN. This is a common issue with any system that uses or can use analog. However, with pre-paid minutes, when the minutes are gone, so is the service until you pay again. But if you have one of the calling plans, the minutes are endless.
As for the minute usage, yes, you have to press the mirror button to end the conversation. I believe it is stated in the Onstar manual to press the mirror button to disconnect the call.