The 237B (1A1) KTU was perfect for this application. Although it was officially branded as an add-on conference unit to tie two pre-determined lines together for conference calling, there's no reason why the many NC and NO contacts provided can't be used for any purpose. The original circuit design provided for it to be controlled by the A lead of a non-locking key (basically a momentary ground) to activate or de-activate the latching circuit.

We often used these KTUs for a night service or ring transfer button on the attendant's 1A2 phone, rather than those silly TKM side car switches. With all of the available relay contacts, we could even use this KTU to busy out the lines in hunt on small systems if needed.

You could certainly use a dial intercom code to control one of these, but you'd have to reverse the polarity of the buzzer voltage input to it. Rather than having the intercom switch the +/-10VAC battery, you'd have it switch the ground and have the battery voltage standing on the other side of the buzzers. All packaged intercom units had provisions to allow for this reversal.

Now finding a 237B will be another subject.

Ed Vaughn, MBSWWYPBX