If you brought the iPhone over from Verizon it may have VoLTE disabled. I would go to your call setting and see if you have the option to turn it back on, if not Verizon may have had Apple remove the menu altogether. It's dropping down to 4G because it is able to do VoLTE, once off your call after a bit it should roll back up to LTE.
Thank you for the much needed response.
On the Settings > Cellular > Cellular Data Options > Enable LTE > "Voice and Data"
Data roaming is ON as well.
On the Settings > Phone > WiFi calling is OFF.
Other than this, I do not see what else can I manipulate to hold on to LTE to make calls. It can be as you said, Verizon buried a special setting !
Fun fact, my Galaxy S8 and S8+ are doing the very same. This cannot be a coincidence right? I called 611 and talked to someone, and they were able to confirm that their Galaxy S8 is holding on to LTE even during a call.
So that makes me question, is this an area thing? I made sure that I have Native T-Mobile coverage and not roaming. The rep also mentioned the fact that I have about 5 tower sites surrounding me and my full 5 bar of LTE proves that. It can be a technical issue in the way that towers handle the call? I couldn't tell. I will take my phone for a drive later on and see how it behaves outside of my home area.
This definitely sounds like a network issue! I would recommend that you Contact Us so that our tech support teams can open up what is known as a "Trouble Ticket". This ticket will go to the local network techs and they will investigate what is causing your VoLTE to fail.