Most cell phone companies have user to user calls with no need for an app
These two options are NOT free:
Callthrough: With Callthrough you can make calls even without internet. App will call a local access number and we will connect your international call at incredibly lower rates.
Callback: You tell us where you want to call, and we will call you back and then connect the call. Callback will save you money in Countries where local access is not available.
Summary: If you are on Verizon and want to call someone on ATT but do not have unlimited minutes, it’s not night, and it’s not a weekend, then this app MIGHT be useful for you. The app is only useful to dial from iPhone app to iPhone app. Many other programs out there to do this, free too, and much easier to use and do not require an registration process.