Skype
I agree with most of the apps listed here but have to say that the small business I work for was able to slash our international calling charges drastically by switching all of our international callers to Skype. We deployed Skype business edition with Skype Manager so that we can apply credit from a single interface and control who gets how much credit. We can generate reports of what calls are made and when credit is applied.
The amount of credit we purchase for our entire group of users is 60% less than what we were paying to our telephone service provider over a 3 month period.