from what i understand your saying everything is unplugged from the phone lines, the phone, and the computer. if there is no coneection at all to the outside world and you still here it, i beleive CheeseBall is right there is a ringer built into your house, you should bother the phone company a lot so they will check it out, they are probly running tests on the lines, that or someones running an old war dialer in the area but defintly get the phone company to come check it out. figure out exactly where in the wall its coming from maybe one night decide to stay awake and wait for it. Good Luck, if its condo or apratment check with landlord about ringers installed.