I've been reading this with some interest, and it inspired some searching and reading. The function seems essential, it's very very strange to see it missing.
I see two options. ACR call recorder, with a helper app, and it still doesn't record the call? Even with the helper app? Or did that get it to work? But it's proprietary blobware deep in the system either way.
Then the other is bcr. I knew nothing about it a little earlier but after reading about it it sounds like a much better choice, source code is available under GPL and it sounds pretty sane. But it's not currently part of Graphene, and of course the devs for the likes of Graphene OS can't just shovel code into a privileged spot in the system without thoroughly auditing it (and likely doing some rewrites) first.
I found several other solutions in my search but it appears all were broken by the latest android release. For some reason someone at google seems to think that just because it's illegal to record certain calls in certain jurisdictions call recording is something they must remove from their android product. But then they also tell you the solution is to use their voip product "voice" which does still allow you to record.
I completely understand Graphene OS doesn't want to support rooting it to put bcr on it, and if it comes to that I'll have to look elsewhere for help, but is there an easier/better solution here at the moment? Would it be possible to add bcr to the OS before installing it to the phone, for instance? Is there even another flavor of android where this does work out of the box right now?