https://ubports.com/en/blog/ubports-news-1/post/first-steps-in-volte-support-for-ubuntu-touch-3768#blog_content

Thank you both for your input. I now see that there is an article on this topic. I didn't know that was the issue before, so thank you for the enlightenment. I really want this to succeed so I will just need to get up to speed a lot quicker on helping support the development. Maybe there is something I can help with on this in the near future.

Re: UPDATED USA: T-Mobile (and MVNO) settings for a fully functional UT device

I recently purchased a google pixel 3a xl and really want to use it as my daily driver. I initially had issues with the apn settings but found the post above and set my settings to the t-mobile configuration. I now have data, messaging, and out-going calls working fine. (although there is a huge delay before actually connecting to the number) My issue now is, I can not get incoming calls to work. But strangely enough, the call will successfully go to my voice mail. Does anyone have any experience with this same set up, and maybe be willing to help me figure out the issue? I am using OTA5 stable.