Cannot enable VoLTE or WIFI calling on new XM purchased iPhone SE

I started using XM with a BYOD iPhone 5c. This phone worked fine.

I added a BYOD iPhone SE, 1st generation, and purchased a new iPhone XS. BOTH phones work fine. 
After receiving an email stating that the 5c did not support VoLTE AND was no longer supported, I ordered an iPhone SE 2020 from XM to replace the iPhone 5c.
The new phone works fine, I can make calls, but I cannot enable VoLTE or WIFI calling. I can enable LTE, but when I try to enable VoLTE,  it just spins for a long while and eventually goes back to switched off.

I contacted support thru chat and by phone. No help. Chat dropped me after I was told to reboot the phone. After a long session with tier 1, then tier 2, the connection dropped. Although she said she would call back if dropped, she didn't. 
The 1st generation SE VoLTE option works properly. I can enable VoLTE. If I swap sims between the two SE phones, the new SE VoLTE OPTION works properly, and the old SE fails to enable VoLTE. 

If I can't enable VoLTE on my new phone, how am I any better off than I was with a 5c - that didn't support VoLTE?

I believe there is some option missing in my account for the one original phone number, because both other numbers will enable VoLTE. Maybe it's on the Verizon side? I have read that VoLTE can be "unprovisioned" by Verizon (although this was about a Verizon customer).

Ideas anyone, please?