Here is a brief video, that describes a Telephony Denial of Service (TDoS) service for hire. The owner shows a rack of centrally managed PCs, each running Skype, and used to coordinate an attack on an enterprise, consumer, contact center, etc. This service would certainly overwhelm a consumer and could be used to prevent the consumer from receiving a confirmation call from their bank, when an attacker is emptying their bank account. This service could also create a TDoS attack on a small enterprise or contact center.
Here is the link to the video:
I found the link on the Krebs for Security site. Here is a link to a blog post with some background information and a little more info on the service:
There are more scalable ways of performing these attacks, but this service is certainly effective.