Default Android voice stream manipulation on custom ROM. Is it possible?

Hello everyone,

I wasn't sure if this topic should be here or at the Android Software Development section, so i'm gonna write it here.

I am interested in building an app, so that voice calls (not VoIP) are end-to-end encrypted. What i want is to modify an existing ROM, in order to implement a cryptographic sheme, so that when someone make a voice call, it will be encrypted, and decrypted at the receiver's android device.
The purpose of course is to keep any phone call safe from the provider. First of all i am wondering if this is possible.
As soon as it is possible, which is the best way to achieve it? Modifying the device's baseband (modem) or interfere in the audio system?
Is there a cryptographic module suitable for this kind of operation, so that information could be decryptable when reaching the other end?

The above project will be used for research purposes, so i don't mind if this kind of manipulation is illegal, yet.

Every suggestion is welcome.