Wifi calling not automatically starting

CM71

Google Pixel 7 Pro, Android 15 (but happened since 12)

I've searched to see if this topic has been covered, couldn't see, and apologise if it has.

Quite often, wifi calling won't automatically initiate, and I have to turn aeroplane mode on then off (or reboot) to get it started. Not the heaviest user, so don't really notice it out and about, but very much do at home. I have a mesh (BT Whole Home) but it also wouldn't work properly when I had Devolo powerline adapters.

Removing Bitdefender from my mobile, and it seems that wifi calling initiates automatically all the time (though it has only been a few days trying, so might just be a coincidence). I don't like not having any protection at all, so am using a competitors free AV and wifi calling seems to work properly, but I'd like to use my Bitdefender subscription. TBH signal is poor where I live, that wifi calling is a must.

Is this a known issue and/or do I need to adjust any settings? TIA