OPTION 1 (free2dial.tripod.com have more details regarding charges)

If you have a O2 contract you can make free international call to the
followings countries as it is inclusive in your free minutes:

1) 0870 794 7861 (Weekday Monday - Friday)

0844 570 7861(Weekend Saturday and Sunday)

2) Enter Destination number including country code and area code

Europe
Andorra, Armenia, Austria, Belgium, Bulgaria, Croatia, Cyprus, Cyprus,
Mobile, Czech Republic, Denmark, Denmark, Mobile, Estonia, Finland,
France, Georgia, Georgia, Mobile, Poland, Portugal, Romania, Russia,
Russia Moscow, San Marino, San Marino, Mobile, Serbia and Montenegro,
Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey Istanbul,
Ukraine Kiev, Germany, Gibraltar, Greece, Iceland, Ireland, Israel,
Israel, Mobile, Istanbul, Italy, Kiev, Liechtenstein, Lithuania,
Luxembourg, Monaco, Monaco, Mobile, Netherlands, Norway,

Americas
Antartica, Antartica, Mobile, Argentina, Argentina Buenos Aires,
Bahamas, Bahamas, Mobile, Bermuda, Bermuda, Mobile, Bolivia, Brazil,
Brazil Rio De Janeiro, Brazil Rio De Janeiro & Sao Paulo, Brazil Sao
Paulo, Brazil Sao Paulo & Rio De Janeiro, Buenos Aires, Canada,
Caracas, Chile, Cochabamba, Cocos Island, Cocos Island, Mobile,
Colombia, Colombia, Mobile, Costa Rica, Costa Rica, Mobile, Dominican
Republic, French Antilles, Panama, Paraguay Asuncion, Peru, Peru Lima,
Puerto Rico, Reunion Island, Santa Cruz, Uruguay, US Virgin Islands,
USA, Uzbekistan, Venezuela, Venezuela Caracas, Guadeloupe, Guam, Guam,
Mobile, Jamaica, Lima, Macao, Macao, Mobile, Mariana Islands, Mariana
Islands, Mobile, Mexico, Mexico City, Mexico Mexico City, Mexico Mexico
City, Mobile, Mexico, Mobile,

Australasia & Asia
Australia, Bangkok, Bangladesh Dhaka, Brunei, Brunei, Mobile, China,
Christmas Island, Christmas Island, Mobile, Saudi Arabia, Singapore,
Singapore, Mobile, Taiwan, Taiwan, Mobile, Thailand, Thailand Bangkok,
Thailand Bangkok, Mobile, Thailand, Mobile, Hong Kong, Indonesia,
Indonesia Jakarta, Jakarta, Japan, Korea South, Korea South, Mobile,
Kyrgyzstan, Malaysia, Malaysia, Mobile, Mongolia, Mongolia, Mobile, New
Zealand,

Africa
Algeria, South Africa, Zambia, Zambia, Mobile, Zimbabwe, Ghana, Malawi,
Malawi, Mobile, Nigeria, Nigeria Lagos,

OPTION 2: (free2dial.tripod.com have more details regarding charges)

For all others services provider (vodafone, orange, 3, T-mobile and
virgin) You just need to send a text message and your phone will
automatically be credited with =A33 worth calling credits. Any call made
will have a 10p connection charges but this is negligible considering a
lot of destinations just cost 1p/min. Furthermore, there is no expiry
date to the credit and no long password to remember, I find it
extremely convenient and the quality of the line is very clear as well.

This is how it worked,
1) Simply text CC to 83006
The text message costs =A33 (charged to you by your mobile provider) and
for this a calling card will be send directly to your mobile phone.
2) Simply dial 0207 124 6666
The number is definitely free in the inclusive minutes from any
services provider
3) Enter Destination number including country code and area code

Below are the countries that me and my friends had tried and cost only
1p/min

Asia Region: Australia, China, Hong Kong, South Korea, New Zealand,
Singapore, Taiwan,
America Region: Argentina Buenos Aires, Canada, USA
Europe Region: Belgium, Denmark, France, Germany, Ireland, Italy,
Netherlands,=20
Norway, Spain, Switzerland, Sweden, UK National