If T-Mobile stopped allowing non-VoLTE phones on its network in January 2021, then we should have seen a ton of angry comments on the internet about it. The lack of a public outcry indicates that T-Mobile is still allowing non-VoLTE phones on its network. It is clear that T-Mobile wants people using its network to upgrade to VoLTE phones, but it isn’t clear when it will stop allowing non-VoLTE phones on its network and whether it will have a strict whitelist of allowed phones or not.
I believe that T-Mobile is putting out these statements so that it has something to tell irate customers when it shuts down a 3G network in their area, but the shutdown of 3G will be piecemeal and T-Mobile doesn’t want to deal with the huge customer backlash of not allowing any non-VoLTE phones on its network. It would overwhelm its support staff and cause T-Mobile to lose a lot of customers to AT&T. Instead, T-Mobile drops scary announcements to get its customers to change their phones, but considering that T-Mobile only announced its change 7 months ago, I don’t think January 2021 is the real cut-off date for non-VoLTE phones. Since T-Mobile isn’t clear with the public about its VoLTE policy, I don’t think that Purism can tell you what T-Mobile is planning.
From @mladen’s email to you, we know that the spec sheet for the BM818 now says that it has VoLTE support, but it doesn’t sound like the MB818’s VoLTE is configured for specific carriers, so Purism is going to have to add software that configures for the carriers. That is how I interpret Mladen’s email, but I would appreciate an official statement from Purism to confirm that.
As for testing other cellular modems which @mladen also mentioned, it doesn’t look like Thales is offering Linux drivers for the Cinterion PLS83-W, so that one seems to be out of the question.