I’m having the same good call quality experience on latest Android 11 AOSP I flashed early of Dec '23.
Just tested now to wife’s iPhone 15 - zero echo or call quality problems, also on speakerphone.
People complained repeatedly about this on my previous X10II, which I think I could take out of the drawer and try flashing a newer AOSP release on it 
as discussed in Anyone flashed Software binaries for AOSP Android 12.0 – Kernel 4.19 – Seine on Xperia 10 ii? - #7 by wetab73
Official docs Updating the Vendor Image | Sailfish OS Documentation
EDIT Well, problem is there are no newer Android 10 releases since Dec 2020, but I think 11+ aren’t compatible on X10II?