Heavily modified GEC (not GPO) 746, that isn't actually a phone as such...

[Jim Stettler]

two different buzzers
[/quote]
My guess is "Caller" and "Unlocked"

[twocvbloke]

My guess is "Caller" and "Unlocked"

Most likely so, though confusing as the PCB that controls them only has one pair of wires, so possibly uses voltage reversal or something to operate one or the other, should have taken pics of both sides of said board really...

Also, the round buzzer I just remembered has three wires, one for High, one for Low (presumably tone) and one common (I think), so adds to the fun... 

[Jim Stettler]

Also, the round buzzer I just remembered has three wires, one for High, one for Low (presumably tone) and one common (I think), so adds to the fun... 

unlock and open?

[Key2871]

The two different buzzers would alert the alert the attendant to the type of signal is my guess.
With out having to look at the indicator they could tell by the sound. Makes sense it was military since all marked components were scrubbed to as you said, eliminate reverse engineering.

[twocvbloke]

unlock and open?

On that I'm not sure, I need to go back in to actually examine that section of the wiring, it could be the little PCB maybe oscillates or switches back & forth between the two tones to create a tone-ringer effect, or not, I don't know, though I'm not sure if it has any transistors to accomplish such a thing...

[Key2871]

Thats possible too...

[Jim Stettler]

On that I'm not sure, I need to go back in to actually examine that section of the wiring, it could be the little PCB maybe oscillates or switches back & forth between the two tones to create a tone-ringer effect, or not, I don't know, though I'm not sure if it has any transistors to accomplish such a thing...

You could get 3 tones
Hi only- Unlock
Lo only-door open
Hi/Lo combo- Caller

[twocvbloke]

I'm wishing I wasn't half asleep right now, that buzzer theory is making me want to go grab the thing and open it up to extract the little PCB to try and see how it is wired up, but I'd probably nod off in the process!!! 

Ah well tomorrow's another day, even though it's now Saturday at 2:47am in the UK... 

[twocvbloke]

You could get 3 tones
Hi only- Unlock
Lo only-door open
Hi/Lo combo- Caller

In a way it could be 4 tones, as there's the large square "12v" buzzer where the gongs would be, which I presume would be the one that buzzes with the door opened, and the other one maybe doing the other theoretical tones, but it's all guesswork at the moment, as I say, need to open it ip again for some examination...

[Jim Stettler]

I'm not sure if it has any transistors to accomplish such a thing...

It could of had a relay box as a "subset".
Just a thought,
Jim

[twocvbloke]

It could of had a relay box as a "subset".
Just a thought,
Jim

That's the thing, there's only two wires to the PCB, which has a few components (inc. capacitor(s) & diodes from memory) that then connects to the two buzzers via the 5 wires to them (two to the square buzzer, three to the round), so there's some electronic trickery happening between the incoming signal and the buzzers...

[Jim Stettler]

Assuming 3 to 4 signals, I can think of these potential signals .
1.Caller-
2.Door release-
3.Door open-  buzz while open

[twocvbloke]

Definitely need to "decipher" the phone to determine its' functions, of course there's this buzzer setup, the lighting, the buttons & their functions, and of course, the audio side, which would be useful seeing as it's meant to be a comms device...

[Owain]

When I saw it I assumed it came from a radio studio. Possibly community or student radio rather than commercial. I had thought that transfer control / assume control (although it's usually labelled offer / accept in broadcast systems) enables the broadcasting studio to offer the transmitter to another studio, and a second studio to accept the transmitter, and then the second studio goes on air, taking over from the first.

But it is possible it is for transferring control of the external door between studios.

There's no way that standard of construction relates to any commercial or military application. It's been made up from available parts, even the mix of ribbon and twisted wiring, the mis-matched body and handset, and the mis-matched screws on the front panel suggest that. The military transmitter probably came from J Bull in Haywards Heath.

[countryman]

There's no way that standard of construction relates to any commercial or military application.

Agreed. The breadboard circuits alone show that it was built individually, maybe by an ingenious student or studio tech, for a certain purpose.