You may have to call your carrier and have them change some things. I had trouble with AT&T a couple times, but then they had me turn off the phone, sent some data to it, then turn it back on. All good now.

Which phone / carrier? I've been using VoLTE on a Pixel 4a with AT&T, and have heard other devices working with T-mobile as well.

Moto g7 plus on straight talk (using att sim I think)

I haven't gotten VoLTE on my XZ2 Compact with Lineage 17.1 and Telenor Sweden to work either. If I have it enabled, I can't do phone calls at all, when forcing 2G/3G calls, at least those work. But I do miss HD Voice.

If your SIM card is really old, some carriers need you to get a new SIM card for VoLTE to work, no matter what phone you have. Also, this is probably a dumb question, but are you on the latest build of LineageOS?

Yeah I update it every week

Device dependent. Too old and volte won't work. Also, Samsung devices typically will not have working volte on LOS and other custom roms.

VoLTE works for TMobile.

Good to know, thank you!