I have 2 Android phone (I can be rooted on them) . I can write on each of them Application. But I have no internet for those phone. I want to send audio file from 1 phone to another via phone call and...

What is the difference between Cellular calls and VoIP calls in LTE. As both use IP packets to send data to EPC. Without internet VoIP calls doesn't work but cellular calls work. Am I missing somethin...