Have you all been living under a rock!
A shared line is where they take a single line and then share it amongst two users. It uses the similar concept of ADSL where the line is broken into wavelength chunks. Like a POTS filter, they install two filters on this single line, thus allowing two people to share without interfering with the other line.
Since the two lines use up most of the available wavelengths, you cannot add in ADSL anymore.
Often shared lines are installed where its too expensive to install more cable (like on farms) or in suburbs where it will take too long, or too expensive to install a separate line. Often they do this on the same property.
Most likely you have a shared line with one of your neighbours. You need to find out who got the line first, though your line might be the 2nd one placed on the existing line.
(We had a shared line 17yrs ago. Originally we had a line installed on a farm cottage we where renting then our neighbour wanted a line too. The then waiting list was like 6-12mths but my Dad jumped the queue due to his health. So we gave our neighbour permission to share our line. Voice quality dropped but never bothered us)