[H] VOIP

[Winterlight]

My home, like most, has a phone network wired in that connects in a 
closed circuit.  That circuit then is connected to the land line 
provider, in my case Quest.  If you disconnect the four wires 
connecting the house's phone network to Quest, then you have a closed 
network or circuit for your phone system.

I have been told that to use VOIP, one needs a special VOIP router 
that connects to my Cableone co-axial cable and it then must be 
connected to the base station of my phone system.  Basically, this is 
an RJ11 cable from the VOIP router to the phone base station.  Why is 
it not possible to connect the RJ11 phone connector directly to an 
open  RJ11 jack in my home phone network and have the existing phone 
system connected into this RJ11 closed home phone network?