For a comprehensive article on how cable modems work go here:
http://computer.howstuffworks.com/cable-modem.htm
Nope. That is dial-up. Cable modems use coaxial cables to transmit data.
Yes, but you should use a drop splitter (2 way balanced) from the input source. Then use the other to connect to addition tvs.
The cable modem can be faulty, check with you service provider
Private line modems worked exclusively with voice and data private lines
Ther are two types of VoIP, one that requires VoIP modem and one that is through web user interface account. It doesn't matter whether it is a cable modem or an ADSL modem.