You are making/receiving the calls with Gnome Calls?
I have a JMP phone number. When I setup Chats with the JMP account as XMPP, I get texts to the JMP number.
I get “Account encountered an error” when entering or trying to use JMP via a VOIP account with Calls. 
The transport defaults to UDP and the Port to 0. Maybe it should be something else?
I know that port is wrong.
So I couldn’t tell from the replies here, whether or not your own personal SIP service should work within STIR/SHAKEN (once it has been fully implemented) or not.
XMPP client port is 5222 and sip is 5060.
So I figured…
OK, bear in mind that I am using Gnome Calls here.
So I tried 5222 and got: Account is offline; domain could not be resolved.
Then I changed the Transport to TCP and got: Account is offline
Later, without doing anything further, I got: Account encountered an error; Internal error occurred.
Progress, I think.
It’s been pretty quiet on the VOIP front, so I would give a short personal update. I installed Gajim and entered a login to my JMP/Cheogram account. It connected on port 5222.
GOOD: I can send/receive messages to other XMPP users… and seemlessly send/receive SMS to mobile phones by adding them contact as “+1<phone#>@cheogram.com”.
ALMOST GOOD: I can make/take voice calls to both XMPP and telephone calls. I can hear the other party 5x5, but they cannot hear me.
TO DO: Test video calls and file transfer.
INTERESTING: I can open Calls, call the phone number associated with my XMPP account and the call loops back to ring Gajim. When I answer, it connects, but of course there is no feedback – because Gajim never patches the mic!
Anyone have experience Gajim?
How about any service other than Giigle Voice?
I wanted to let everybody now that I ordered and received a JMP.chat physical SIM. It works very well. I do not use it as my main cellular provider (I use Mint-Mobile) on the Librem 5. However, my experiments with it show the sim for JMP.chat to be very reliable.
This is cool because I can have WIFI calling and Cellular calling all on one number with JMP.chat.
You can even add multiple JMP numbers to your account (with additional subscription costs), and have them all on one device (and on all your devices). It’s a cool service.
You will have to report back if it continues to work without bugs, because that is very tempting 
I’m using JMP.chat plus their data SIM (or an alternate data-only roaming SIM) as my main mobile “carrier” now. (Not yet on the Librem 5, though; I’m still using degoogled Android.)
Calling works seamlessly over the internet with no need for VoLTE, using either the native Android dialer or their Cheogram app (available from F-Droid). (Other XMPP clients can work as well.)
“SMS” and “MMS” (including group messages) are handled by the Cheogram (or other XMPP) app.
Unless I delete chat histories, my messages follow me to whichever alternate phone or computer I want to use.
JMP can’t call the short-code emergency services, but the phone itself can, even without a SIM card. However, emergency services can’t see a call-back number for you, so that’s something to be aware of.
P.S. I don’t know if all the kinks have been worked out to use XMPP like this with the Librem 5, but if not, it’s probably close to operational with the right XMPP application.
P.P.S. As stated elsewhere, JMP only provides Canada or U.S. phone numbers (new or ported in), but calls and messaging will continue to work as normal no matter where in the world you travel, as long as you have an internet connection.
Another advantage of using a roaming data SIM in conjunction with XMPP and/or SIP is that the SIM often can use more than one national network to provide service. For instance, in the U.S., the data SIM can connect to either T-mobile or AT&T, dependent on which one has available coverage in a particular area, thus bypassing AT&T’s restrictive device “whitelisting,” because it’s a data-only SIM. And obviously not requiring VoLTE to work, as I mentioned above.
Of course, you can also use XMPP or SIP in conjunction with a regular mobile SIM, and have both the phone number from the service provider, and an additional phone number or numbers from, e.g., JMP or a VOIP provider.
Add a dual SIM phone into the mix, and you can expand your options.