While not free, many of the VoIP services like Voipbuster, etc. do offer rates to mobiles that can be a lot cheaper than calling from a traditional landline - they are usually eliminating the international dialling portion, where the phone companies normally make some further margin.

For example, calling a UK mobile from voipbuster is EUR 0.18 / minute. Calling this number from a US landline could easily be more than $1 / minute, so there are some big savings to be had.