Telcos often have everything good in their CO switch to provide disconnect supervision, but they are using pair gain equipment to reach distant areas. Although the open loop signal is sent by the CO, it is not being repeated by the pair gain card(s) due to their individual settings. You can test this with a butt set while monitoring the line when a caller hangs up. If you hear the audible click of the line going open for a half-second, but your polarity light doesn't wink during that same time, then it's definitely the pair gain settings. Good luck in getting them to admit this, but it happens frequently with ALL service providers and ALL switches.

Ed Vaughn, MBSWWYPBX