There’s a good explanation of the technology here.
I’ve been using JMP on my custom Androids for a while, and it works very well. I also got one of their data-only SIMs (in beta), although I’m testing it in one of the Androids, not in the Librem5. I suppose I should try it in the L5, although I don’t anticipate any problems; it’s only a physical SIM card after all. (Soprani can provide an eSIM instead, but that would not work with the L5, at least in its current state.) The SIM service uses European carriers for cost-effective roaming here in the U.S. and Canada. Roaming in other countries is not available, but may be added later.
I think the basic JMP.chat service is available to anyone anywhere, but one would only be able to choose a U.S. or Canada number, of course.
EDIT: I tested the JMP SIM card in the L5. Instead of using the Orange France APN (which did not appear without manually entering it myself), it defaulted to AT&T after a minute or so. I ran a speed test; this is from inside my house:
purism@pureos:~$ speedtest
Retrieving speedtest.net configuration...
Testing from Express Teleservice Corp (89.31.243.3)...
Retrieving speedtest.net server list...
Selecting best server based on ping...
Hosted by Trunk Networks (London) [7293.78 km]: 316.874 ms
Testing download speed................................................................................
Download: 8.22 Mbit/s
Testing upload speed......................................................................................................
Upload: 1.04 Mbit/s
purism@pureos:~$
FYI, that’s a server in Massachusetts, whereas I am located in California.