Which model of the S7? Some models I see are fully compatible, some are partially to not compatible with T-Mobile's bands.
If you purchased a non-T-Mobile S7, it probably won't have WiFi Calling, so if you need WiFi Calling, that probably won't be the device for you.
I'm assuming you're not on WiFi when you're making these test calls. I think this has something to do with VoLTE. There was a post recently about a Samsung device (maybe was the S7?). VoLTE implementations appear to be carrier specific, so you may need to disable this in order to get it to work. One test you can do to determine if it's a VoLTE problem is set your phone to 2G Only mode and test. You'll then have to go into the settings to see if there's a way to disable VoLTE (different carriers / phones have different navigations).
As for the OS version, gsmarena says it's upgradeable to 8.0. You'll have to talk to Samsung as to the appropriate way to get it updated to the OS (it's not a T-Mobile branded device, so T-Mobile can only provide basic support, and this does not include OS updates). Some devices don't update while on cellular, so try keeping the device on wifi for a while and check. You can also try Samsung Kies to perform the update.
thanks for your response, smplyunprdctble . I'm not sure how we did it, but we got it working. It's as if it needed a while to get up to speed. Anyway, we didn't have to tinker with the settings to be able to make calls, but I'll save this message in case we have problems later.
As to the update, Samsung says that t-mobile has to give us the update. Whatever. I'm sure I could spend hours going back and forth getting the runaround from both Samsung and t-mobile. That's the kind of thing that makes me want to go off the grid completely, I hate it so much. The phone is for my son; he's just going to have to live with 6.0 and maybe if he's lucky an update will show up on his phone one day.
thanks again for your help.
T-Mobile cannot push an update to a device that doesn't have their OS installed.
I still question whether or not they have the ability to push an update to device, but that's another story.