Android :: Best Way To Call With G1 While Traveling?

I have a G1 phone and I am very often traveling outside the US. I am a little confused with the solutions that exist for using Wifi access to call while outside the US. My question is: what is the most cost efficient way to call with my G1 while outside the US using access to Wifi and without roaming? A typical scenario would be that I am in France or in the UK, and I want to call a local number or a cell phone number in either of these countries but don't want to roam to avoid huge bills. I have access to Wifi most of the time when in France or in the UK.