I have already contacted you via PM 12 hours ago, please check your inbox
it is a service company, and im only interested in how they have achived this kind of filter that is all.
i do not think calling them by GSM to ask them about thier security system will be any reasonable for them, that is why im trying to figure out what they have done.
OK, calling this number (assigned to O2 mobile but obviously isn’t a mobile) with Voxbeam Platinum (call routed via BICS), gives a 603 Declined when I sent my Paris DID. I then spoofed a public Munich number (not mine) and heard:
Herzlich willkommen bei ihrem handwerkerdienst wir sind 24 Stunden für Sie da. Zu Verifizierung ihrer Rufnummer rufen wir Sie jetzt zurück.
Of course, I had no way to receive the callback or even determine whether they actually attempted one.
It is not surprising that this type of company would reject international calls. I further suspect that their system checks the caller ID against a list of known customers, and anyone failing the test receives a callback for verification. I’m not familiar enough with the German system to know whether one can look up the carrier of a geographic number to determine whether it is VoIP or not.
If you would like me to test this further, PM your German geographic number (along with a good time to call) and we can see whether you get the callback.
Maybe I’m the only one, but I see a difference between having a problem you are trying to solve and asking the community about it (apparently on several forums), and having an academic interest in how some other company is doing voip blocking. You set up this pretense which is annoying and a waste of helpful people’s time.