Does anyone sell an inexpensive plug-in device that verifies the presence or absence of DSL service on a line? I've got a Harris butt set with data lockout, but I'm not sure I trust it. Sometimes I go across a line that has DSL, but get no warning signal from the set, then a moment later clip on again and get the beeping warning. I'd like a simple device that lights up or gives a visual indicator that DSL really is there. The only things I've found are complete testers that cost a fortune. Anyone?