if the line tests OK sounds like the levels are down
you need to connect to the socket where the outside wires come into the house with ALL the internal wiring
if you have an NTE5 ...
http://www.bttorj45.com/BTsocketmasterNTE5.htm l
just unscrew the lower front and use the test socket behind
this makes sure the rest of the house is disconnected (you can plug a "real" phone into the filter when you are sure all's well.
to be certain you need to leave it like this for a couple of days.
if it's still iffy ... you can argue that it's faulty to