http://mobiletalk.packet8.net/

P8 provides VoIP services to consumers and small virtual offices. Now they have added software (which works on Palm o/s phones) to allow cell phone users to make international calls at decent rates.

I have used P8 for around 3 years and found their service extremely reliable.

It's worth a read if you make a decent number of international calls while away from your home, or office IMHO.