i was wondering too. How hard is it to aquire a re-seller account of pre-paid mobile service, and then "Port" the number.
I dont know much about "number porting", (for those non-usa folk, that is moving you phone number to a different company/carrier. or house number changed onto cell....)
at the very least your gonna need the fulll info.
I never ran across this problem, and worse case scenerio, just use phone flooding service to flood the number with calls, and put your pre-paid/voip number as 2ndary, and they will have to call 2ndary.