Creating a basic VoIP using Swift

What are the basic requirments of having a simple phone app using using a phone number to connect connect to one another.


I just want to create a basic app where I cam cimmunucate with my app using two of my iphone. where for example Phone 1 # (917) 555 - 1234 dails Phone 2 # (213) 666 - 4321. How do I registar phone numbers?