On landlines on some networks, it used to be that one person on the call hung up but the other didn't, the call would remain connected for a while. So if the person who had hung up, picked up the phone again, they'd still be connected to the same person, and wouldn't hear a dial tone. This was useful: if you wanted to move to a different phone connected to the same line, say, for more privacy, you just tell the other person what you're doing, hang up, and then pick up in the other room. But some scammers found a way to use this.
Basically, the scammer would instruct a suspicious mark to hang up, look up their bank's phone number, and call back, just as Krebs is instructing. As soon as the mark hung up, the scammer would begin playing a dial tone instead of hanging up. When the mark picked up the phone, they would hear the dial tone, so they would begin dialing at which point the scammer would end the dial tone, wait for the dialing to stop, and then play a few ring tones and then pretend to pick up the phone. This was in the 90s so the technology was there to automate it, but it's simple enough it could have been done completely manually by stopping and starting recordings. From the user's perspective, it seemed they had hung up and made a call to the bank, so it seemed impossible that they were connected to someone else.
Cell phones, which don't stay connected when only one party hangs up, are totally immune to this hack. And this hack has probably fallen out of use since the ubiquity of cell phones has deprived scammers of a viable pool of marks.
Basically, the scammer would instruct a suspicious mark to hang up, look up their bank's phone number, and call back, just as Krebs is instructing. As soon as the mark hung up, the scammer would begin playing a dial tone instead of hanging up. When the mark picked up the phone, they would hear the dial tone, so they would begin dialing at which point the scammer would end the dial tone, wait for the dialing to stop, and then play a few ring tones and then pretend to pick up the phone. This was in the 90s so the technology was there to automate it, but it's simple enough it could have been done completely manually by stopping and starting recordings. From the user's perspective, it seemed they had hung up and made a call to the bank, so it seemed impossible that they were connected to someone else.
Cell phones, which don't stay connected when only one party hangs up, are totally immune to this hack. And this hack has probably fallen out of use since the ubiquity of cell phones has deprived scammers of a viable pool of marks.