I followed https://forums.ubports.com/topic/8338/usa-t-mobile-and-mvno-settings-for-a-fully-functional-ut-device (which was linked in the post you gave) and even on 2G, it's just ending the call seconds after hitting call. (Maybe 3 seconds)

I did do a reset of all apn settings before adding the apn settings and then restarted the phone after adding the settings listed there. Sending texts work, but I can't receive any. Calls not working.

Could this be area related? This is the identical outcome I had with my pinephone when I tried using it with UT. (And any other distro)

EDIT:
I also saw the part of the post where it talked about not having two entries with the same APN. I did see where the default MMS entry used wholesale, so I deleted it and restarted the phone to see if that would work and it's still the same.

Device: Pixel 3a
Build: 20.04 OTA-2
Carrier: Mint Mobile

Made it through the install of UT and now I'm running into an issue making or receiving calls as well as sending images (texting works fine). I've been searching most of the day for a fix, but have been pretty stuck.

I created 2 apn settings.
Internet:
Name: Ultra
APN: wholesale (I tried Wholesale as well)

MMS:
Name: Ultra MMS
APN: wholesale (I tried Wholesale as well)
MMSC: http://wholesale.mmsmvno.com/mms/wapenc
Proxy Port: 8080

I've actually tried some other settings as well, but these actually let me send/receive texts, so I figured I was closer with these. I've also tried locking to 2G, but it didn't change the outcome. Currently I've got it set to 2G/3G/4G. I've also liberally restarted my phone after most setting changes.

Interestingly enough, every time I try to call out, I see the icon for the cell network drop and then come back before saying the call has failed.

I hope this is the right place to ask about this. Thanks in advance for any help!