The telephony denial of service (TDOS) attack is a way to divert your attention from what's really going on, and a way to make you unavailable to banks and other financial institutions.
According to NJToday, the scheme works like this: cyber thieves have somehow managed to obtain your account information. They get in touch with the institution where you account is open, change information such as phone number and email address - or even bank account numbers, then keep your phone line busy and prevent the institution from checking up with you and verifying the changes and confirm transactions. When they do manage to get in touch with you, it is probably too late - your account has been emptied.
The FBI has recently started warning the public about a recent resurgence of this king of TDOS attack. The scheme isn't new - the FBI was notified for the first time about this kind of attack back in November of 2009. Back then, the attacks were traced to a slew of VoIP accounts connected to automatic dialing programs, but the perpetrator(s) has remained unknown.
Usually, the victims don't realize that something suspicious is going on - they think it's a glitch with their carrier. The best thing to do if you are on the receiving end of such phone calls is to alert immediately every institution you have an account with and tell them to block it, then contact the authorities.