One more vote for the pay-as-you-go phone idea, if you're in countries where it's available it's a great way to go. The phones themselves are usually dirt cheap (my last one was $10), and if you're clients aren't calling you every day the phone doesn't cost anything while it's not being used.
Traditionally the cheapest way to make international calls in a lot of countries has been at the post office, this might be a cheap way for you to call your clients should you need to.