The #'s are definitely something that shouldn't be shared.
If you are id'ing a particular line just call your cell from the line and hang up before you hit voice mail and you won't use any minutes.
If you are trying to look for a particular # on a demarc frame that has exposed binding posts, a good trick I learned was to call the # you were looking for and run down the frame with a piece of copper shorting out pairs, when you hear a crackling in the phone you are calling the # with, you hit the right pair.
