sounds like the phone line is just a coincidence, i think you have a loose connection in the VGA cable so you're only getting one or two colour signals. try another screen/cable you know to be good, or replace the cable if poss.
the usual cause for this is when you have a loose connection between the pc and the monitor if it's a desktop change the cable and reseat the video card if you have one. if it's a laptop it's a little more fiddly. and will involve checking the connection this usually involves removing the top plate and keyboard but this will vary from make to make, it may need a new connecting cable.
