This is a brief whitepaper that discusses DTMF input processing and easy denial of service attack via phone lines against banking systems.
You can download it from the following link: https://packetstormsecurity.com/files/download/116622/rahulSasiPaperDetails.pdf
Source: https://packetstormsecurity.com/files/116622/How-I-DOSed-My-Bank.html