It is a common belief that if you answer a robocall that your phone number will be “remembered” and the volume of calls you receive will be increased. This theory has even been promulgated by the FCC website.
It turns out, a study by North Carolina State University provides compelling evidence that this belief is not true. The researchers there set up a set of over 60,000 telephone numbers connected to computers to measure the volume of robocalls. The phone numbers used for this study were never given out to anyone. Some of these numbers were never answered. Some of them were always answered. The strategy for some was changed over time. The result, after a year, was that answering these phones or not made no significant difference in the number of calls.
To me, this study suggests that the robocallers just call every phone number they can.
In any event, I would add “Answering a phone call from an unknown caller number increases robocalls” to the list of unfounded urban legends.
I got this information from Crypto-Gram, a monthly email newsletter from Bruce Schneir. Here’s what his article said
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[2020.08.17] A group of researchers set up a telephony honeypot and tracked robocall behavior:
NCSU researchers said they ran 66,606 telephone lines between March 2019 and January 2020, during which time they said to have received 1,481,201 unsolicited calls — even if they never made their phone numbers public via any source.
The research team said they usually received an unsolicited call every 8.42 days, but most of the robocall traffic came in sudden surges they called “storms” that happened at regular intervals, suggesting that robocallers operated using a tactic of short-burst and well-organized campaigns.
In total, the NCSU team said it tracked 650 storms over 11 months, with most storms being of the same size.
Research paper. USENIX talk. Slashdot thread.
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You can can find the entire newsletter here:
https://www.schneier.com/crypto-gram/archives/2020/0915.html
You can find links to the NCSU paper and other information in the quoted excerpt above.