t1me in regards to first party AOSP solutions a quick search of the forum throws up this previous discussion:
https://discuss.grapheneos.org/d/111-status-of-call-recording/11
While the response there may be blunt I agree that this would be a nice feature to have.
That said, as GrapheneOS is focused on security and privacy improvements first and foremost, this may not be possible to implement at the moment.
GrapheneOS does aim to be highly usable as well as secure, so we thank you for pitching in with this idea!
Keep in mind that the project could also use more resources in order to be able to facilitate more requests like this.
Please consider donating. If everybody pitched in, the project would be able to have more full time developers working on all kinds of new features!
You can find out more information about donating to the project here:
https://grapheneos.org/donate
If you can though you can try build the OS yourself and integrate those patches using the instructions here, and if you find it easy enough to maintain then you can suggest doing so: https://grapheneos.org/build
For third party solutions I know Josh of Side of Burritos showed an app called Cube ACR.
That is not a recommendation nor endorsement from me, I don't use it, but simply pointing you in the direction of a possible alternate solution for your own consideration.
https://www.youtube.com/watch?v=FEN_NvKsVBc