Can the SIM card itself play a role in this?
the one I’m using is a bit weird.
It’s new, (bought two or three days ago), and it’s from Austrian Budget provider/MVNO “S-Budget Mobile”.
Now what’s weird about it is that they claim to use T-Mobile’s network (called Magenta here), so MCC 232 (Austria) and MNC 03 (T-Mobile).
BUT.
The APN/MMC/SMS configuration that was applied to the Settings app is that of tele.ring (MNC 07).
Actually precisely the settings given in mobile-broadband-provider-info/mobile-broadband-provider-info/serviceproviders.xml at 4a1617295360c10331e748e3a9f42311fbe3325c · sailfishos/mobile-broadband-provider-info · GitHub
Also, if you use their website https://www.s-budget-mobile.at/, while the text makes it clear in many places that the network is T-Mobile/Magenta, the website itself makes a lot of connections to telet.ring named services.