TT Products used to be located in Yonkers, NY and they supplied many different gadgets for the interconnect industry until they went out of business a few years ago.
www.Sandman.com has this product that does the same thing as the TT unit. Model # CID2N for $26.95 each.
There is no reason that you can't use one relay for multiple sets, because the accepted wisdom in these cases is that probably only one telephone, on each line, will be off-hook at one time.
If you just want the relay to activate the A-lead control in a 1A2 system, then you'll be OK. If, however, you want to take a line off HOLD that has been put there buy another user on the system, you'll be SOL unless you wire the T & R of the set(s) AHEAD OF the line card, rather than BEHIND it.
Refer to this thread for the explanation:
https://sundance-communications.com/forum/ubbthreads.php/topics/488492/A_Lead_controlIf you can tell us what types of phones you want to use, there is more info that we need to share. For one thing, these simple devices (TT and Sandman) do not have slow-release relays, so using them with rotary sets will cause the 1A2 line lamps to blink when a single-line set is dialing.
If you plan to use conventional 500- or 2500-type sets, there is a method that allows you to convert the sets for A-lead control, and avoid the blinking and HOLD problems.
