@moem
The OPO can use LTE for data, but without VOLTE support in UBPorts, voice calls will still be carried on 2G or 3G (when and where these services still exist).
If you're using the OPO with UBPorts somewhere/sometime that 3G and 4G are coexisting, you can watch it happen: Pull up a web page using mobile data, and you'll see the "L" icon in the status bar on the top. Now, place a voice call and see the "L" replaced with "3G". This is the device falling back to 3G since there's no OS support for VOLTE. End the call and "L" comes back.
T-mobile's announcement of 2G and 3G sunset for their US networks says the T-Mo 3G service ends July 1 2022, and this is the one we care about for the OPO. On that date -- unless you're somewhere T-Mo is still supporting 2G for legacy hardware -- you won't be able to make calls. But depending on where you are, you may get cut off sooner since Big Pink is decommissioning 3G stations as we type and you'll be out of luck . . .
This has already happened to me with an AT&T reseller. 3G service on that network ended Feb 22, but had been degrading before that date. OPO now sits in a drawer unless I need to play Tux Racer. 
BTW, UBPorts (and other alternative OSs like Sailfish) will work -- kinda -- for VOLTE-only networks on the Pinephone since that device has VOLTE implemented in hardware in the modem.