How Does A Robocall Blocker "Know" That I'm Getting A Robocall And Not A Real Call?

3-4 times per week I get robocalls from “The Security Company,” always from a spoofed number. I’ve thought about installing a robocall blocker, but I’m concerned it would block legitimate calls, too. What’s the mechanism that prevents this?

The call ID is used as a lookup into a database. If it matches a know spammer, it gets blocked.
That’s why the system isn’t foolproof - if the number hasn’t made it into the database yet, the call goes through.

They have a constantly updated database of numbers to work with. So, if you answer a call that turns out to be spam, you tell the app that it was a robocall. After enough people do that in a short enough amount of time their algorithm adds the number to the black list and it’ll block calls from there on out.

I don’t know the details, but what I’ve read says that beside the database of spoofed numbers they look for patterns - so if a certain number is supposedly making many calls a minute, it is suspicious and gets blocked even if no one reports it.

Which is also not foolproof, so calls get through - just not many.

Some systems also use “dialer detection”, which is looking for the dead time between when you are connected, and when the message starts, characteristic of auto-dialer systems.

I don’t know how common this form of detection is now.

I used nomorobo for a while, and suspect callers weren’t just blocked; they got a message that their call was suspect, and they were offered a chance to enter a number to prove they were real. So, legitimate calls could get through.

I’ve given up completely, and just don’t answer any call from a number I don’t recognize. I trust that if it’s someone I actually want to talk to, they’ll leave a message.

I do that manually. If I answer the phone and ther is a dead time, I hang up.

(But that’s not foolproof, either. I once hung up on a friend who happened to get distracted by her kids just as I answered the phone.)

My landline phone company installed NoMoRobo for us this week. Not every call gets blocked - there’s a surprisingly stubborn company that wants desperately to talk to us about Medicare-approved braces.

I also understand it’s possible to unblock certain number, so an automatic reminder from your pharmacy will go through.

I don’t see how a spam number database could help. All your better spammers are spoofing numbers now. I get calls every day from spoofed numbers that come up with a person’s name on caller ID. Caller ID name data isn’t transmitted by the caller; the system just looks up the number and serves a name. Most of the time they have the same area code and prefix as my phone.

My Google Voice number has an option for screening calls; caller must announce and the announcement is played back for me before I accept the call.

I get plenty of calls from the same spoofed numbers over and over, enough that I can either block them on my work landline or I at least recognize them and don’t pick up.
If that spoofed local number gets tagged as spam or the app sees it calling hundreds of people an hour, it’ll start automatically blocking it. So far as I know, the app isn’t interested in the name attached to it, that’s just a bonus for the robocaller, it’s just looking at the number that shows up on the receiver’s end.

I suppose if a robocall DOES get through, you could press a button on your detector before you hang up to add it to the database?

I use Skype Out to call people in America. A lot of these services just really hate Skype. It can be very difficult to get through.

Carriers (in the US) are starting to implement verified Caller ID, where they flag calls that have a caller ID that they know is true (i.e the call originated on their network and the caller ID has not been spoofed). When they are able to validate this flag between trusted network interconnects, spoofing will become far more difficult, as those numbers will not have verified Caller ID.

My Pixel 3 also allows me to screen calls. The Google assistant answers the phone, relays a message that they are being screened and to state their name and reason for calling. I see the whole transaction in text on the screen. If it is spam, that number is then blocked from calling me.