VoWifi is also not working on the FP3 (Germany Telefónica) with latest Iode 2.4 build. IMS is registered however VoWifi not available, although turned on in the settings. I switched from FPOS where it worked
Its enabled and greyed out for me as well and working (was the same with stock ROM). What does IMS service say (see three dots)? You can do a test call and see if mobile data remain at 4G.