The service is guaranteed to work and it's not that expensive
Webroot experts have found that cybercriminals are offering “phone ring flooding” services that are advertised as being capable of disrupting the competition’s phone lines.The service, which has been in operation for 3 years, is similar to SMS flooding and it’s guaranteed to work.
It can be successfully used not only to “take care of a competitor’s phone line,” as it’s advertised, but also to launch denial-of-service attacks against the customers of banks and payment processors in an effort to preventing them from learning about fraudulent transactions.
The cost for such services varies on underground markets, but it’s not very expensive. For instance, if someone wants to take out a phone line for a maximum of one day, he would have to pay $3 (2 EUR) per hour per number.
For longer periods, for instance between 2 weeks and 1 month, the service costs $25 (19 EUR) per night per number.