Recently I came across http://www.truphone.com
Truphone website. What they are offering is free calls from your mobile via WiFi. Their software only works on Symbian phones and not even all of them - just Nokia E-series (N-Series will be supported soon as they claim).
At the moment they offer free calls in the UK and some international destinations
Argentina (Buenos Aries only)
Brazil (Rio de Janeiro only)
China (Landline & Mobile)
"Hong Kong (Landline, Mobile & Pager)"
"Mexico (Guadalajara, Mexico City & Monterrey only)"
Poland (Warsaw only)
Russia (Moscow Central & St Petersburg only)
Singapore (Landline & Mobile)
US Virgin Islands
USA (Landline & Mobile)
Vatican City State
These destinations are free until e/o this year.
It sounds quite good (apart from phone limitation, of course) but have anyone actually tried their service?