I wouldn't worry about 1 milliamp, as long as your phones ring when they should -- that's a very low number, and the voltages are normal, so the line is still seen as on-hook. If you want to find out what's drawing the current, it would be easy to just go around the house and disconnect whatever is plugged into that line pair, one jack at a time, and see when the current draw goes away. It's probably one of your devices (phone, all-in-one printer/fax, fax machine, etc), using that on-hook current to power its circuitry.