@Flohack thanks for the clarification. Is there any minimum requirement on the LTE UE category, or is the main roadblock the IMS support at the software/firmware level?

@robthebold fwiw, AT&T said my service would have continued uninterrupted through 02/2022 if I hadn't swapped the SIM

@clessley the issue is that LTE networks were rolled out for data usage while 3G networks remained in use for calls and SMS, but now, US carriers are dropping support for 3G networks (T-Mobile 01/2021, AT&T 02/2022), and thereby phones which use circuit-switched fallback.

Early LTE devices have used LTE frequency bands for data transfer, but "fall back" to 3G UMTS frequency bands for calls and SMS. This was provisioned for since LTE doesn't support circuit-switched calls, and this is why you can't use data on older phones while making a call.

I've used a Nexus 5 since last September, but it got water-damaged so I bought a Oneplus One. When I switched SIM cards, my AT&T account got suspended: they have done this since January as part of a phase-out leading up to 02/2022.

I believe the solution is to purchase a phone which is compliant with LTE Category 6, or above, since it will use the IMS services necessary for supporting VoLTE (T-Mobile) and HD Voice (AT&T).

@lakotaubp done, thanks

@flohack thanks for the suggestion, the following solution worked for me via https://forum.xda-developers.com/t/rom-ota-mirrors-for-official-cyanogen-roms-and-ota-updates.2906746/ and https://forum.xda-developers.com/t/guide-unlock-root-flash-for-oneplus-one.2839471/

image="cm-13.1.2-ZNH2KAS3P0-bacon-signed-fastboot"
wget https://ava4.androidfilehost.com/dl/ouITqWz89SZhokgspXAnMw/1620431504/24591000424960109/${image}.zip
unzip ${image}.zip -d $image
cd ./$image
fastboot flash modem NON-HLOS.bin
fastboot flash sbl1 sbl1.mbn
fastboot flash dbi sdi.mbn
fastboot flash aboot emmc_appsboot.mbn
fastboot flash rpm rpm.mbn
fastboot flash tz tz.mbn
fastboot flash LOGO logo.bin
fastboot flash oppostanvbk static_nvbk.bin
fastboot flash boot boot.img
fastboot flash cache cache.img
fastboot flash recovery recovery.img
fastboot flash system system.img
fastboot flash userdata userdata_64G.img
fastboot reboot

Then after I set up Android and enabled developer settings + ADB, the ubports-installer 0.8.6-beta appimage worked as expected

(1) I received OnePlus One A0001 Cyanogen with Android
(2) I changed Android settings: enabled developer settings, enabled USB debugging
(3) I got the UBports installer
(4) I booted OPO to bootloader, ran fastboot oem unlock from terminal, then rebooted to recovery
(5) Now I open the UBports installer and it recognizes bacon
(6) I select stable/wipe/bootstrap, then run
(7) Installer reports success, then OPO reboots
(8) OPO goes to an "Installing Updates" screen with a Yumi animation, finishes and reboots.
(9) Now sometimes:
(9a) It goes into boot loop: display lights up, but goes off before the UT splash screen.
(9b) It gets to the UT setup prompts, but if I remove my phone from the computer, the boot loop occurs again.

I've tried UBports installer 0.8.8-beta (snap, deb, and appimage)
I've also tried the 0.8.6-beta appimage.
I've tried running the 0.8.6-beta appimage from the terminal as root.
I've also tried the halium-devel channel.
I've also tried several different USB cables.

Any recommendations would be appreciated, thanks!