Creating a situation where one user can dial an escape digit such as 9 to access a single trunk is easy. if all you want to do is to demonstrate the concept, a few relays will do it. However, if you know how a PBX actually sets up a call, and you think about how that call progresses, you will need to rethink your project.
Every extension in a PBX has a relay that identifies its line (The L relay) and another relay that cuts it off from being called when its busy (the CO relay). 100 extensions would have 100 L and 100 CO relays.
In your case, if you wish to have (only one) single-line telephone (SLT) pick up the dial intercom (DICM), dial 9 and receive outside dial tone from a pool of just one CO line, the ext will need to have some way of holding the CO line until the call is ended, and some way that allows other users to access the DICM for intercom calls while this other connection is still up.
You will also need to wire EVERY extension that wants this service with its own loop-sensing relay OR wire all the single-line sets that want this service for A-lead control.
The sequence would look like this:
EXT off hook on DICM
"A" relay operates
DICM waits for digit
EXT dials 9
One-second pulse from DICM unit operates "9" code relay
"9" code relay cuts through CO dial tone to extension
DICM unit restores to on-hook condition
"9" code relay locks up on its own windings, under control of the "A" relay
.
.
.
Call ends
Ext hangs up
"A" relay falls
"9" code relay falls
In this scenario, the other users of the DICM will be able to use it for ICM calls, but anyone of them can dial 9 and barge in, unless you equip all of them with their own Cut Off relays.
The reason you need to provide either loop sensing (with a conceptual L relay) or A-lead control for every member of the group that can dial 9 is so that when the call is finished, the system will restore and release the CO connection for the next user. This is called "supervision" and is the reason that in a real PBX, the truck circuit provides supervision via the "S" lead (Sleeve lead.) In a real PBX situation, the sleeve is grounded back through the switch train, and is under control of the trunk loop current, so that when a trunk call is retired, the sleeve ground is removed and all the parts restore for the next call.
So...at a minimum, to demonstrate the concept, you need one SLT wired for A-lead control, and two relays, one with one set of transfer contacts and one with a single transfer contact.
If you want to use a pick-up on a key set for the DICM, you could use the A-lead control for the key set, but would still need two relays per "EXT" that have access to the demonstration circuit.
If the trunk you want to use also appears on a button of the keysets, you would need to ground the A-lead of the associated line card, so that the line lamp on all the sets with that line would come on to indicate that the line is BUSY. Any user with that line on his set could barge in, BUT you could add a third relay (per trunk) with two transfer contacts that would exclude the T&R of the truck from the key system, so that the dial tone would be cut off from access on those key set buttons.
It would also be advisable to swap the SIG output supply of the DICM unit so that it sends a GRD pulse instead of a BAT pulse. The code relay would then lock up on GRD rather than BAT and the same GRD could be used for grounding the A lead of the line card.
If you can't find a pair of 17B KTU's to use as the relays, and you don't mind using COTS equipment (Commercial Off The Shelf) use two of
these
