Unfortunately I will be bothered by PING calling from abroad and O2 will not be able to block those calls (despite their side's recommendation).
So what to do?
Is there an APP that blocks all foreign calls?
Or can I even do this myself through the harassment filter option of my mobile phone (Huawei P8 lite)?
And if you turn here?: https://www.bundesnetzagentur.de/DE/Sachgebiete/Telekommunikation/Verbraucher/verbraucher-node.html
Possibly. Does it help, if you disable the so-called third-party provider via your provider. I did that and can't get these calls.