www.voipcheap.com are offering free calls to landlines via your computer to Europe, USA, Canada, Australia, Malaysia, Hong Kong, Singapore, etc. The only time limit is 300 minutes per week. That should satisfy most people!

I have tried it out locally and the quality is good and the delay is not noticable. I do not know how good it would be on a dial up connection, but there is an option to select dial up as your connection type.

The registration email received from Voipcheap says that free calls are restricted to 1 minute per call, but my call was over 3 minutes, and a friend made a 14 minute call to Malaysia. She said the reception was better than using Telefonica's own line. It appears that the 1 minute limit is only in certain countries.