Banks use phone calls to alert customers of potential fraud — so the fraudsters have responded by tying up their victims' phone lines.

Cyberthieves are able to pay $5 an hour or $40 a day for a service that makes "junk calls" to a victim's landline or mobile number with the intention of tying up the lines so that a bank can't get through, Brian Krebs reported Tuesday at krebsonsecurity.com.

The tactic isn't new, but Krebs wrote that its transformation into a service is significant. "Everything about [cybercrime] can now be outsourced to third-party services," he wrote.