Id like something like jmp.chat but self hosted, there are a good few USB modems that would let me do this, just all the software makes use of hosted phone number providers.
Could you please tell us which ones?
All of the linked XMPP-phonenetwork bridging services appear to be United States + Canada only, so I have no hope of trying or testing this software.
The best I have in my country (Australia) are SIP providers. They generally only offer landline numbers; I think some might offer mobile numbers but I have not tried those services (they cost more and I suspect texting won't work anyway).
Nonetheless some simple self-hosted SIP-XMPP bridge software would be amazing. We'd also need XMPP clients that understand this, however, otherwise using existing tel://xxx address books would be fiddly (you would have to manually retype them to be an XMPP address).
N.B. SIP clients on phones seem to be a bit slow and unreliable. I use one daily nonetheless, along with Snikket (conversations), which also has its fair share of issues on different people's phones.
I haven't tried it myself (I don't even run an XMPP server anymore) but based on the description, combining something like sylkserver[1], perhaps with a dedicated SIP service (Asterisk/FreePBX?), should get you VoIP phone calls from XMPP. You can probably get one of those SIM-to-SIP devices (or maybe there's software) if you want mobile phone calls.
There's one problem with this setup: emergency numbers may not work and might get routed to the wrong place.
There is another major problem which is latency. I hate it when someone decides to call me using whatsapp by convenience (the phone/call button is easy to reach right on top of the message view) because latency is way too high which means people keep speaking at the same time unless you use it like a walkie-talkie and say "over" when you clean the line.
I know the latency is already a thing with regular wireless phone network and we will probably never get back to analog wired level but it is still much lower than the latency the instant messaging apps call have, which is just too much to be bearable for anything else but the very occasional call.
For the phone network part of this you can use JMP from Singpolyma:
JMP seems to give you a number for incoming. I have no need for that, I would need outgoing into the European phone network.
Edit: Calling to German landlines is $0.015 / min. That's a competitive price as such. But the monthly fee doesn't make it overly interesting for my usage pattern.
I am not actively following this field, but I had somehow gotten the impression XMPP is a dying protocol. Any opinions on that?
I have also looked for VoIP providers offering competitive per minute rates to European phone numbers with no monthly fee. I did not find much. VoIP seems to be somewhat popular in Germany and there is no lack of providers. But their contracts include a German phone number which I don't need and cannot even legally get because I don't live there. I don't think SIP vs XMPP would matter for me.
>but I had somehow gotten the impression XMPP is a dying protocol. Any opinions on that?
Not at all. Have a look at https://snikket.org for example. This let's you set up your own XMPP server that "just works" in a couple of minutes, and then you can hand out invites to your friends and family, so that they can install the Snikket app (Android or iOS) and are automatically registered and in your contacts in a few taps, without them even needing to know what XMPP is.
The server is the well-known Prosody, dockerized, the Android app is a re-skinned Conversations, the iOS client is based on Siskin IM.
And just like that you have a working, private, self-hosted, end-to-end encrypted cross-platform messaging system that also support a/v calls out of the box.
It really is pretty cool.
IETF Standards don't get a large marketing budget assigned to them like many other messengers have, but there are quite a few community projects which are actively developed. Off the top of my head there is Conversations on Android (gratis when you get it from F-Droid), Monal on iOS, Gajim on the desktop, Dino, Movim, Profanity, converse.js, Prosody, ejabberd, ...
For me I had XMPP accounts with zero usage until the Freenode IRC network imploded, then a few channels moved to XMPP and I joined a couple more. That said, IRC is still more active than XMPP for me.