Is it possible to intercept voice data during call?

Question

I plan to develop Android application which will encrypt/decrypt voice data such that parties can communicate using secure channel. So the question is: how to intercept voice/au

Answer 1

In GSM it is not possible to encrypt the voice and send it through the voice channel due to channel characteristics. The signal sent through the voice channel has to be (similar to) human voice as vocoder (CELP) is used for channel coding.

So even if you had access to low level libraries and used it to encrypt voice to send it directly through the voice channel it would be not possible for the called party to receive and decrypt the signal.

Encrypted voice however could be sent through the data channel as data.