Attackers are now using DDoS services that offer attacks on telecommunication systems as part of larger attack schemes. These attacks, known as TDoS attacks, can be a relatively cheap option for cybercriminals looking into diversifying their attack vectors. The Federal Bureau of Investigation looked into the flooding of phone lines in 2010 after investigating an incident where a Florida dentist lost $400,000 from his retirement account through a diversionary TDoS attack. Researchers have seen a series of advertisements and forum posts promoting services that can flood both mobile and stationary telephone lines.
Source: https://threatpost.com/firm-sees-more-ddos-attacks-aimed-telecom-systems-073112/76865/