20+ year old phone systems had the ability to show
when a line was in use, something that asterisk is
completely lacking. I am offering a $50 bounty to
the first person to get an adsi phone's soft keys to
appear as 6 different lines with descriptions like
free, on-hold, incoming, in-use.

A separate $50 bounty if method works on standard
CID phones. For instance if you can look at a callerid
box and can see that 010110 means lines 2,4,5 are in
use. In this case, you might need a special dialplan in
order to select a line.