VoLTE support for the OnePlus 6

I'm opening this issue to document discussions had with @dylanvanassche about VoLTE support, especially in the US, for the OP6.

DylanVanAssche | twiclo: caleb well there are some modem low level config files. Those need to be uploaded to the modem and activated depending on the provider. Besides that, we would need to reverse engineer the QMI messages for the IMS (VoLTE) service. I just bought today a SIM with VoLTE capabilities which is known to work on
               | Android SDM845 devices. This way, I can try out some stuff when I have time without looking for a capable SIM. Don't
DylanVanAssche | expect anything soon though... There's not really a bug bounting program I think.
        twiclo | Well please let me know if you need help with testing this feature. I have a mint mobile sim which works off the tmobile network out of the US
DylanVanAssche | twiclo: ah the US :p well I'm Belgium where VoLTE is not really a requirement yet, it is only available for a few high end devices to test it out. There's some ModemManager issue about VoLTE, like what woule be needee from the Purism people but that's mostly it
        twiclo | Yeah here in rural Utah regular calls are practically impossible
DylanVanAssche | If you want to play around with it, you need to use qmicli with the --pdc-* args and upload/activate the right PDC profile for T-Mobile.
DylanVanAssche | These profiles are stored in the firmware vendor partition on Android
        twiclo | Okay I'll have to get in touch with you later on when I'm able to boot my OP6. I'm having issues with that at the moment: https://gitlab.com/postmarketOS/pmaports/-/issues/1874
        twiclo | How do you want us to track the progress/how should I get in contact with you? An issue sounds like the way to go here
DylanVanAssche | You can access them on pmOS by mounting one of the modem partitions I think
DylanVanAssche | However, this stuff is risky...
DylanVanAssche | PDC profiles may cause issues when done wrong. I already had such issues :/
DylanVanAssche | twiclo: Sure, you can make one on the pmOS pmaports issue tracker and tag me
DylanVanAssche | I plan to look into it eventually... Currently making progress on the sensor side, they are hard to get them running :/