Beware the non-telephone company versions of A-lead control devices. Most are simple series relays, and geared toward phones with tone dialing. They are seldom engineered adequately to prevent relay chattering (and therefore lamp flickering) during pulse dialing.
You could also replace each of your 400-type line cards with a Melco M-487 if you can find them. They allow a single-line set to be wired to the output T & R and the KTU detects when the set goes off-hook. I'm sure Geraldo...er...Ed...has a few in his vault. I have two, but I'm hoarding them for my museum display. They originally cost over 200 bucks, as I recall, and could only be special-ordered by the boss.
I prefer using the A-lead conversion, though, because you can install an eentsy N/C pushbutton on the front of the 554 housing in series with the A-lead, and by pushing the button as you hang up, you can put the line on Hold, allowing you to take the call on one of the 2565's.
