TL;DR: Android 11 fw with Android 12 binaries gives the best result in my test.
Proper solution which will not require reinstallation if SailfishOS was installed ontop of Android 11 base.
I got the best result by installing the Android 12 binaries ontop of SailfishOS installed on an Android 11 base as the solution mentioned below result in no waiting tone while calling people using VoLTE.
First of get the Android 12 v2a binaries here: Software binaries for AOSP Android 12.0 – Kernel 4.19 – Lena (v2) | Developer World and extract them.
If you’re sure your phone ran Android 11 prior to installing SailfishOS you can simply power if off, holding Volume Up while connecting the USB-cable and the flashing the Android 12 binaries using these commands:
fastboot flash oem_a SW_binaries_for_Xperia_Android_12_4.19_v2a_lena.img -S 512K
fastboot flash oem_b SW_binaries_for_Xperia_Android_12_4.19_v2a_lena.img -S 512K
fastboot reboot
If above commands fails on Linux/Mac you will need to add “sudo” at the start of each command. Also make sure you have “cd” to the correct directory containing the extracted binaries (one file).
I’ve tried lots of stuff, eg. Android 12 base with Android 11 and 12 binaries which neither way gave me wifi. Do yourself a favor and use Android 11. It’s worth the hassle setting up a Windows machine with Emma as at least in my case all quirks seem to be gone (broken VoLTE, unable to send sms with VoLTE, missing waiting tone with VoLTE and possibly also echo issues in non-VoLTE calls)