It looks like in Germany they are phasing out 3G this year (Vodafone, O2) but keeping 2G available at least. For now…
One, admittedly rather geeky solution, may be to get yourself a data-only SIM and run SIP on top of that.
That’s basically what I’ve been doing for quite a while now.
You forgot the link
https://openrepos.net/content/unmaintained/s1p