First of all, I would like to say thank you. Without this website and all the contributors providing us free guides, I probably would have to get rid of both my LG G8 and G8X for expensive Apple or Samsung phones.
My phone is Sprint unlocked LG G8X running Android 10, software update G850UM20p.
I can confirm that simply replacing both abl_a and abl_b with the same modified abl_a version from https://forum.xda-developers.com/t/...nlock-and-magisk-root-using-firehose.4221793/
like Nate0 did allow both VoLTE and Wifi Calling (maybe Video Calling too) to work with TMobile without doing anything else.
I simply used Partition Manager in QFIL to backup both abl_a and abl_b, closed out of Partition Manager like the guide say (I'm not really sure if this is absolutely necessary or not) went back into it again and loaded the modified version of abl_a from the link in the guide into both abl_a and abl_b. I did this with an inactive TMobile sim inserted.
When I rebooted the phone, it now displays TMobile logo instead of Sprint. It also installed TMobile Hotspot and TMobile app. Also gone from Network & internet in the settings menu is the option for toggling VolTE and Wifi Calling on/off, but there is now a standalone Wi-Fi Calling option that you can toggle on/off. The hidden menu *#*#4636#*#* now shows provisioned for VoLTE, Wifi Calling, Video Calling. The picture showing Video Calling as not provisioned is from Sprint LG G8 cross flashed with US Open firmware G820QM20f_00_NAO_US_OP_0402.kdz.
I tried cross flashing the G8X too using a guide written by netmsm, but couldn't get it to work for some reason although the same method worked perfectly for the G8 (thanks netmsm) kept getting error 0x6004, CrossDL TMO_US>OPEN_US. Both VoLTE and Wifi Calling works after cross flashing the G8 running 20f update with US Open firmware but dont know why Video Calling is not provisioned like on the G8X.
I tested VoLTE and Wifi Calling on the G8X with an active TMobile sim and both worked perfectly. Calls now connect instantly and sound clearer and not take 5 or more seconds to switch network like when VoLTE wasn't working.