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I've used callback services years ago. When I was in South Africa a $5 card from http://www.thephonecardstore.ca/ would give me as many hours (12, I think) of talk time to Canada as I would get minutes by just calling the regular way. You can still buy these and they seem to work fine. You can also often get cards at convenience stores but I can't read all the terms and conditions under those cirumstances.
That said, I've been 100% Skype on foreign trips for a long time now.