This comes the closest to the issue I've run into, with a 9900 from 2011:
purchased 9900 unlocked, but not from a carrier
went to use it on T-Mobile, worked fine but no Wi-Fi calling
Spoke to T-Mobile, who said to update it to 7.1 for Wi-Fi calling. Worked fine on cellular band. But couldn't load OS 7.1. Then found your other post, John, mentioning deleting the vendor file on the PC.
That let me load the 7.1. But it still doesn't permit voice calls on Wi-Fi - the 9900 OS 7.1 just does data on Wi-Fi.
Pressing ALT + CAPS + H shows Vendor ID = 612
But this isn't T-Mobile's Vendor ID.
I'll try to see if the last comment you made will let me change the Vendor ID, but I've heard that this can't be done with the newer OS versions.
Any suggestions for easier way of changing the 612 Vendor ID to T-Mobile's Vendor ID?