LTE & call waiting?

[Tcantcan]

I'm on an xperia xz2 compact. It only works on verizon via lte. A tech turned on the feature in my account so I can receive texts. Couldn't do that till they turned it on. But I keep getting calls and texts where I get no notice, display or sound, and just stumble on the text or missed call later. Sometimes with a voicemail left. It seems this is either entirely or in part because call waiting isn't working. When on one call there is no notice another is trying. I found something, not sure if true, that call waiting doesn't work over LTE. Anyone have more info? Is there anything else verizon needs to turn on on the back end? Oh, and the setting to turn on call waiting is grayed out and can't be turned on.