Here are a couple of articles about various DDoS services set up and ready for hire. There is also a brief discussion about how a service was used to attack a VoIP service. This isn't TDoS per se, but as I have said before, the botnets that the attackers use could easily generate VoIP based calls (rather than lots of packets) and target any enterprise site, contact center, user, consumer, etc.
So a tiny bit of background here. Attackers create botnets by assembling a collection (100's, 1000's, 10,000's) of individual bots on zombied computers. These bots sit dormant, but are ready to go if the attacker wants to generate an attack (or is paid). The bots can do pretty much anything - often it is to generate specific or floods of packets for DDoS attack. The more bots, the more traffic and harder it is to mitigate the attack. Of course the bot can generate one or more VoIP/SIP based phone calls, which could for example all hit a large contact center. If the bots are smart enough to spoof their caller ID, the would be difficult to differentiate from real traffic. If they play similar audio, that could be detected, but if they did something really nasty, like play a random audio file/song from the zombie computer's iTunes directory, the attack would be very difficult to mitigate.