Its not faulty, and its happens quite a lot not just with Virgin Media and auto dialers, So much so that Ofcom has rules regarding this.
http://www.ofcom.org.uk/media/mofaq/...ms/silentfaqs/
Basically the system may ring 100 people at a time, with 20 staff waiting to take the call. obviously your not going to get the full 100 people answer, but if 21 people do answer, if your number 21 your going to get a silent call. Obviously I've just made the above figuers up, but the companys will have some sort of stastics that they use, and most of the time it shouldn't be a problem but you can never get it spot on all the time.
Reading the link above the calls shouldn't be silent and should have a recorded message.