Another variant using Skype is their SkypeNumber or [online number](http://www.skype.com/en/features/online-number/) service (formerly called “SkypeIn”). Unlike Skype's regular voice-over-IP service, there is a monthly fee but there are several advantages:

* Overall costs are still reasonable compared to traditional long-distance/international calls
* It's completely transparent to the caller, they just call something that looks like a regular local/national number, no computer is involved
* You pay for it, depending on their phone contract it might even be completely free to them

Caveat: The terms of use are (probably deliberately) vague on potential residency requirements. In some places, it might conceivably be illegal to get a number without living there. I assume being able to provide a US address or US credit card would be enough in practice, if even necessary (I personally never tried to get a number in the US).