After trying all the remedies I could find on-line I finally came up with a real one working with a GREAT T-Mobile support person.
T-Mobile offers a CellSpot router (TM AC-1900) that will fix the problem.
I tried it and it did eliminate the problem.
It is basically a customized Asus RT AC68U.
The modification essentially gives WiFi calling the highest priority for bandwidth on the router.
I already had an AC68U and wanted to use it instead.
If you have an AC68U, N66U, or other router that has a Traffic Manager and/or QoS (Quality of Service) settings, you can do the same thing with your own router.
Go to your Traffic Manager.
Turn QoS on.
Go to User-defined QoS Rules.
Enter the following two rules giving them a meaningful name like "WiFi Calling", enter the MAC for your phone, enter at least 85% of your available bandwidth (e.g 0-42500 if your maximum transfer rate is 50 Meg), the highest priority and:
Rule 1: Destination port "4500" Protocol "UDP"
Rule 2: Destination port "5060, 5061" Protocol "TCP"
Doing this I went from very repeatable ER081/082 to no problems at all.
This does not really help if you need wi-fi calling outside home. I need it at work, can't play with the wifi there...
Enabling IPSec passthrough fixed this issue for me