I may be barking up the wrong tree here, if so forgive me,
I thought mobile phones worked in the same way irrespective of which country you are in and who your tele-communications' provider is.
I can block all unidentified calls to my phone. By unidentified I mean a person whose name and phone number is not typed in by me in my list of contacts.
Once this is done, the phone simply does not ring when anyone else tries to get through.
This saves me a lot of annoying phone calls and limits the risk of being caught out by a scammer.
It may well be that some are clever enough to get round this precaution, but most are not. I think they just hear the dialing tone when they ring me, so they do not realise I have blocked unknown callers.
If I need to be able to accept a call from someone not on the list, I can either remove the block on unidentified call until I have heard from them, or more usually, I just add their phone number and company name to my list of contacts.