Looking for Cheapest International Calling from Stateside Mobile Service

So I'm looking for the cheapest mobile service that has some form of international calling from stateside USA (need more than just Canada/Mexico), there are many countries I need to make calls to any given day for work, and I know each service usually has different number of countries supported for international calling. But the most important thing is not the number of countries available. It's actually if the mobile service actually allows large amounts of international calling, in the past I had issues with mobile providers where after about 2 hours of international calling they flag my account and disable the so called "Unlimited international calling", which I find crazy. Any ideas on a good service for my needs ?