I´m also living in Germany and using blau.de (Eplus) on my Sony X Compact with newest SailfishOS 4.4.0.64. I experienced the ofono-cpu-guzzling since the first version i using that Device (4.1).
It seems it occurs once a session and after perform “systemctl restart ofono” it doesn´t seems to happen until warm/cold reboot.
It´s also hard to find what triggers this. There is another Thread here in the Forum where we try to find the reason for it Ofonod CPU guzzling