@zilladog I too am a little late to this forum post. I just stumbled upon this as I too am trying to figure out what to do with my Xperia X. My recent experience with T-Mobile suggests that VoLTE is required.
I used a Sony Xperia X (F5122, dual SIM version, with an outdated Sony-supplied Android) with T-Mobile in the US for about two years (2018-2019), and then I moved to Canada, where I've been using the same phone for the last two years with a Canadian operator. I had also cancelled my old T-Mobile plan.
When I visited the US last month (November 2021), I bought a pre-paid T-Mobile SIM, assuming that the phone will continue to work with T-Mobile. I could even activate it at the store, using the same phone. But an our or later it was dropped out of the network. I tried switching SIM slots, rebooting, and so on, but none of that worked. So I called customer support (from another phone, of course) and learned that the Xperia X is probably no longer supported. I suspect this is because T-Mobile had dropped support for phones that do not have VoLTE, such as my F5122, by August 2021 or so.
https://www.theverge.com/2020/7/23/21336397/t-mobile-require-support-volte-2021-5g-lte
You can check T-Mobile's support status of your phone by entering its IMEI number in their website, here: https://www.t-mobile.com/resources/bring-your-own-phone
It seems that certain non-US versions of the F5122 Android firmware have a toggle switch that can probably enable VoLTE, but I do not want to go down that particular rabbit hole. Perhaps the next time I will give UBPorts a try!