Any T-mobile-based MVNO should approve the BM818-A1 for activation:
Of course, if you don’t enable VoLTE, which hasn’t been officially implemented yet in the -A1, and which I think is still experimental, you won’t have reliable (or possibly any) calling capability.
I hear Tello is quite cheap, but whether the call/SMS/data buckets/vs/price fit your particular needs, you’ll have to judge.
See also
https://source.puri.sm/Librem5/community-wiki/-/wikis/Cellular-Providers
There’s maybe also the JMP.chat + Gajim route: VOIP with jmp.chat and gajim