You want to scramble your voice before it's sent through the phone? In this case you'll need a descrambler on the other end, and the quality will probably suffe

You want to scramble your voice before it's sent through the phone? In this case you'll need a descrambler on the other end, and the quality will probably suffer since voice codecs are lossy and you'll have to account for that in your algorithm. It sounds like a fun project but practically speaking why not just use some end-to-end encrypted VoIP protocol? The only practical use case I can see for your solution would be to do it over normal GSM calls when you don't have a data connection. Then it could have some value.