I managed to get the open source call recorder working on a rooted LineageOS device. Seems it doesn't work without root.

Beware - recording people without their explicit consent is illegal in lots of countries.

You could also use an office SIP phone and/or a telephony server (PBX) and a landline which might include recording capability. Or something like Google voice or a business phone provider which might offer email transcripts as part of their service. Or you get the audio via an (emulated) headset. Either cable or bluetooth. But that requires additional hardware. And check if your default dialer has some recording feature. I can enable that when opening the default dialer in the settings. And while in call a "record" button will appear next to "hold". It's not automatic though. You have to start a call and then manually record every one. And it might play a beep sound to the other party.

Recording would be the first step. Without that working, we don't need to discuss the next steps. But I agree, the next steps would be to chain something like Whisper or FasterWhisper and then an LLM.

I suppose the next flagship android devices (Pixel?) come with similar features, as Google etc are pushing their AI services and features.