https://grapheneos.org/features#other-features

Call recording functionality within the Dialer app using modern Android storage with recordings stored in Recordings/Call Recordings and no restrictions based on region or special cases like playing a recording tone (users are still responsible for complying with their local laws).

Says there’s no region block, so not sure why it wouldn’t be there for you.

Edit: the button only shows up when a call connects & it’s on the bottom right.