Vomitorius but how is this possible, if I use my cellular data to browse internet but cannot call?
Your assumption that cellular voice just goes over cellular data is wrong.
It's not only possible, but was the default roll out for 4G in most carriers in most countries. Specifically 4G was originally rolled out as LTE data only, voice was still running on 3G and would fail over to 2G is there was a fault or lack of coverage on 2G. It simplified things operationally and for support. Not only did most carriers not support voice on 4g, most phones didn't either. Which was fine until carriers needed more frequency bandwidth for 4G and 5G and started cannabilizing 2G and 3G frequencies for 4G and 5G data. Turning down all 3G service (both voice and data) is not uncommon. Some carriers have even turned down all (non-emergency) 2G service as well. Sometimes it's a carrier probem. Often it's a MVNO that can't keep up with changes that the MNO (carrier) is making in its infrastructure. There are still incompatability issues between voice over 4G or 5G as implemented on some carrier equipment and as implemented on some phones. And then there's provisioning systems that only allow voice on 4G on an individual contract or phone basis because they want to reduce customer complaints and support costs. And then there's GrapheneOS which by default blocks 2G for security reasons or you might never have noticed this in the first place.
I've simplified the explanation. The real world is much more complicated and messy.
But what can you do? That depends largely on who your provider is and what options you have or are willing to entertain. Simply turning off 2G network protection is the simplest thing to try. If your current provider is a low cost MVNO it's likely that you'd have better service switching.