I use technology called SIP using a SIP switch from a company called Applico Security Inc (
www.applicosecurity.com). This is how it works. I have sim cards from local countries I visit and use Nokia phone wich has CDMA/GSM.
If you call my US number, our office switch forwards the call to the country I am in (our office there) and then PSTN GW in local office dials me locally so it becaomes a local call.
I can use the same system for video conference (multi-party too) using my laptop and I make calls from my Smartphone using WiFi client over SIP.
PM me if you need more details.