Could your line at the exchange be touching another or loose?
I once went into an exchange and the board where the lines came in was a total mess. I remember the engineer saying working on one it was very easy to knock another. Things are probably better now but you never know.
I saw my cab open the other day and took a pic, which I can't find right now but I remember asking the engineer how he could know which is which, it was that bad, his reply was " it's a f***** mess"