This is an excellent question that no content on the Purism website and FAQ, the Forum, or the Wiki seems to be answering clearly. The only confirmation that I’ve seen in this Forum or the Wiki FAQ is that Librem 5 owners with the BM818-A1 modem (phone batch version unclear) have been been able to place 3G calls on the AT&T network. Also, as Amarok points out, the BM818-A1 is not capable yet of supporting VoLTE on the Librem 5.
I received an email from Mladen Pejakovic, Support Manager at Purism, on 01/31/21 confirming the following when I pressed Support about my concern that T-Mobile has announced that they are no longer allowing non-VoLTE phones on their network as of 01/2021, and AT&T has announced that they will no longer allow non-VoLTE phones on their network starting 02/2022. This explains why nobody has confirmed in the Forum or Wiki FAQ that they are using a Librem 5 to make calls on the T-Mobile network, and why Librem 5 users are still able to make calls on the AT&T network, at least using 3G. Mladen wrote, numbering his responses to certain questions of mine:
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BM818 modems support VoLTE (you can find its spec sheet online to confirm).
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We are in process of developing software to enable VoLTE on Librem phone.
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We will also consider other modems for use with Librem 5 in the upcoming months.
So, Purism is working to get the BM818-A1 to support VoLTE. In the meantime, I assume that the AT&T network is now the only network in the U.S. that will accept a Librem 5 for calling, and the clock on that functionality is ticking down till 02/2022. In addition, Purism is working on alternatives to the BM818-A1. Mladen apparently either meant the PLS8-US or possibly other alternative modems.