My husband and I are heading off to the US and Canada in a few months, and was wondering if anyone can give some advice on purchasing a pre-paid SIM card that would work in both countries. Previously we have purchased two pre-paid phones but they only work in USA and the credit only last for 90 days before you lose any credit you have. We both have iphone 3GS and would like to be able to use them. I would appreciate any advice.