Like many people on FT, I travel quite frequently and have a collection of 10-15 sim cards from different continents. Although I have my US T-Mobile number which rings world-wide, it is much cheaper to receive calls on a local number. The thing is that, aside from my cirtica 1999 Virgin Mobile UK SIM which has not yet expired, most sim cards have a shelf life just slightly longer than most boy bands. For the UK and HK, this is no problem because I visit frequently...but for other countries (i.e. CH, MX, etc) my travel patterns are not so consistent. As well, many of my US-based friends do not necessarily understand my nomadic lifestyle.
It is much easier to give people on US number that always works, with the side benefit that they do not have to pay for the international call from their mobile phone. My solution: I have Skype In with forwarding turned on. This means that people dial my US-based Skype number, and it automatically forwards to whatever prepaid sim card I might use in the world. The only downside is that I have to pay for the international mobile phone call...hope this helps.