Similar to the virtual postal address setup, we are looking to keep a virtual US phone number while being overseas. Reasons for this are to give our friends and family in the US a permanent local number to call, no matter where are at a particular point in time. Having a US number for outbound calls is can also be useful to call US based 1-800 numbers - e.g. to call a credit card company and yell at them to stop sending spam to our vitual mailbox for example...
The simplest way to do that would be to get VoIP phone service from a provider like Vonage and the simply pack up the VoIP adapter and US phone and install it wherever we are - using the proper power converters.
However, we are doing something slightly more convoluted. Even though Skype became famous for offering free computer to computer calling using a peer-peer, the now also offer gateways to the plain-old telephone service at a fraction of the cost of most other VoIP providers.
For $9 per month, we have unlimited calling to the US and Canada plus two phone numbers, one in the US and one Switzerland. We can also set up forwarding to any number (free in the US, about 2c/min. anywhere else with the current subscriptions) so that any Skype calls, wheter from a computer user or from a call to either of these numbers will be forwarded to our designated home phone if we are not running the Skype anywhere on a computer.