Developing a Voip application is a pretty advanced topic to be honest. You have to know quite a bit of programming as well as understand the underlying technologies involved with the devices to make everything work properly. This isn't something that you just choose to do, but you study and work hard to accomplish and often there are teams of people doing this.
However, that being said. The basic concept of a VOIP service is that you're taking voice input from the devices microphone, the device reads that into data, your voip application would take that data and transmit it to the appropriate end-device, and that data would then be output into the end device's speaker. There would then be full duplex communication between both devices so that people could talk back and forth at the same time.
In terms of programming this means that you would have to use the appropriate programming language for IOS, you would also likely have to program a server to act as the middle man in order to arrange the call and link up.
It's by no means impossible but a good place to read about this in depth and perhaps come to your own understanding about it would be Apple's development site located https://developer.apple.com/library/ios/referencelibrary/GettingStarted/RoadMapiOS/.
