Does the one telephone have a filter on it, a little beige box plugged into the jack, and then your phone plugged into that. If it doesn't, you should have a splitter installed outside. Outside your house somewhere there should be a box marked Telephone, probably next to your circuit breakers and cable. Is there another box in there, or is it just a bunch of wires screwed onto something. Paladin please correct me if I'm wrong about the cable question.
Street lights have one of three types of ballast.
Each creates an electromagnetic field that transforms the current for discharge lamp use. Electronics produce it, and some electronics are shielded against emitting, receiving, or accepting better than others. It can disrupt some pieces of equipment, such as your comp, or DSL Modem. If you want to see the effect of EMI's. Take a large speaker magnet to a poorly shielded monitor and watch the screen distort or discolor.