I have both a German mobile number and landline number, and have never once received a scam and spam call on either in the 8 years I have been living in Germany. I guess it is a problem of having a public contact number on a website.
Depends on the prpvider as well. Vodafone seems to have (had?) zero qualms about passing my old landline number to multiple shady call centers after taking over Unitymedia. The number was not known to anyone but my parents and I opted out of erverything I could find, still for some time got weekly calls. I haven't had that woth Telekom so far.
Spam calls (and even more so: SMS) have gotten much more frequent on my mobile number in the last 2 years. But in this case, it's a number that has been in active use for the last decade and has by now probably been in multiple data leaks... :/
