Wait a second. You've had voice over LTE disabled for the past year? That's the problem right there if so. A lot has happened over the past 1+ years with their LTE...unless I misread your post?
No, you read it right. Back about about a year ago I was having problems because LTE coverage was spotty where we lived, so they advised that I don't use VoLTE. It did help with dropped calls back then. It has been this way since... I didn't even think about it at first. I talked to another person, mainly to pay off the hardware to give me the flexibility to move lines as needed. She was great, actually. She ran through a few ideas with me before I port out, so I listened.
I made the changes and things are much better today. We'll see how things go next week because weekends are fairly quiet. I'll post back, so hopefully others will find it helpful as well.