The option to silence unknown numbers in iOS13 has killed their annoyance for me. Soon though they'll flood voicemail boxes making just as annoying to figure out u missed something important or not. If that happens then captcha for voicemail needs to be a built in option. Though for now they arent a bother.
I have a landline for business reasons, and that thing gets a dozen robocalls a day. The telco's voicemail apparently has trouble understanding that two seconds of static followed by a hangup is not worth saving.
The cell spam is relentless. I even get three calls a week (always from new, different numbers) to my iPhone that leave messages in Mandarin. I don't speak Mandarin. I've never done business anywhere that would warrant that.
Your cell number is from an area likely to have a lot (i.e. 5%, maybe) of Chinese folks (especially visitors rather than permanent immigrants), it's a scam aimed at them and you're just among the collateral annoyed. It is easy/cheap/etc. enough for the robocallers to just hit all the numbers in your area in hopes of a few bites from those who speak Mandarin and fall for the equivalent of the IRS scam in Mandarin.
You can install an anti-spam filter on your landline by connecting a modem on the landline. A modem allows you to control the phone-line programmatically: if the number is unwanted, just let your program to hang it up. Software like NCID (http://ncid.sourceforge.net/) provides a solution for interpreting the Caller-ID of the incoming calls.
Even better, because the modem only rejects incoming calls, it won't interfere with your phone. You can continue using it as usual.
Of course, a graph where vertices are phone numbers and edges are "likely has in contact book" is probably fairly straightforwards, if not cheap, to buy...
