We're out of the country much of the time and haven't yet found a true single number service that is affordable or offers real worldwide coverage. It involves four figure wireless bills or to carry a few handsets to get even spotty coverage. What we've done lately is to purchase quad band GSM phones and use pay as you go accounts in the regions. We've dvreased cost more than 80% and increased the availability of regions where we can use the phones. I use Orange in Europe (same number for a year) FidoNet in Canada and TIM in Central/South America. We've also used Telestial for other parts of the world.
If I were you I'd hit a local provider and get a SIM and number. It's more reliable and far less expensive than using international roaming.