As the encryption is between terminal and MSC (mobile switching center) you can't by self change the encryption method because MSC will not understand it and will not be able to provide voice to your partner in conversation.
Of course you can use data channels (data trafic) and creating own application to implement desired encryption method. But to establish communication with your partner you will need a lot of infrastructure
P.S. You can't establish own radio communication as this will involved usage of specific radio bandwidths which are (usually) under government control
P.P.S. Some frequencies are free for personal usage (2.4GHz for example) but you are limited by law (usually maximim of 100 mW) so you will have relatively small coverage for calls