I only got the 5.1.1 update last night, but wifi calling has not been stable at all for me. After I enabled it, it took a few reboots and error messages about needing my carrier to activate wifi calling on my account before it actually showed wifi calling in the lock screen or quick settings menu. And for a couple minutes the quick settings display kept changing rapidly between T-Mobile, T-Mobile wifi calling, no service, and emergency calls only (I have terrible service at my house, hence the no service message in there as well). It was like my phone was having a fight with my router or something.
Eventually wifi calling turned on and worked great, but it turned itself off after a couple hours. I got it to turn back on after awhile, but then the phone rebooted itself when I put it on a charger for bed and wifi calling didn't turn back on all night. I still had it turned on in settings, it just wasn't working.
Hope this gets sorted out, or I'm gonna have to switch carriers and pay a lot more than the $30/month plan I'm on now.