Is it even possible on android? Is there a FOSS dialer to optionally encrypt some phone calls (non voip) using a pre-shared key with other party?
Signal is a great example of this but I don't think you'll find any ways to do it non-VoIP.
Instead of fiddling with the limitation on Android set by Google, I think a custom crypto DAC/ADC would be far eaiser, though you need both hard and software to accomplish this. It also came with the added benefit of not processing cryptographic operations on a black box.
Still, I don't know what goal you want to achieve and threat model is. If you are just curious if this possible, the answer will be ye with tons of hops amd hacks. If you really want security, I will advise you go another route.
Just curious really, would be cool using cellular network with an encrypted signal. Here some telcom companies offers infinite calls minutes but limited GBs of internet, so making voip calls would use those GBs.