Not all countries are expensive to call from with prepaid SIMs. UK-US is about 20 pence per minute on Orange and Virgin Mobile. In Australia I believe it was 30 cents (Australian) per minute. So it does depend on the country you are in. In other countries, Mexico perhaps as an example, the Hopmobile or some other option such as GlobalStar might be good. At least in Mexico City with the SIMs I found inbound calls werent free so you were basically tooled anyway you did it.