IMessage comes from a different (unknown) phone number? Possible to change own number

contactsmessages

I finally talked my mum into updating her iphone to iOS5, however now there is a hitch.
When she sends a message to me (also on iOS5) it does come through as an imessage, but from a totally different phone number.

It seems like the phone number is attached to the sim card in her phone even though she elected to keep her old number when she bought the phone. Does that make sense??

Is there a way to change the phone number associated with an iphone?
When I go to settings>messages> receive at the phone number box is greyed out.

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