The Gmail feature extensions  was launched in August 2010 and is based on VoIP technology Google acquired when it purchased Gizmo5 in 2009.
Now in its third free year, there have been several rumors - backed by patent filings - that Google may support the voice calling feature via ads, but it seems that this will not be the case for now. International calling rates range from $0.02 per minute, for example to Australia and Denmark (landline), to $2.00 per minute to Antarctica and $6.90 per minute when calling Inmarsat satellite phones.