"T i m" wrote
| So, the PTM switches 'switch' 12V DV (looking at the bunch of wires
| that are connected in what looks like a home made PSU box with a std
| 12DC PSU in there). So that might activate a relay in the door entry
| panel and that in turn switches the 12V AC through the door release
| solenoid?

That's not uncommon, and avoids having the lock current running through all
the internal wiring (the lock is comparatively high current and sensitive to
voltage drop)

| I haven't traced (or measured) the PSU feed to the camera but it could
| well be AC. I have some pro CCTV cameras here that run from an
| low(ish) voltage AC supply and hence the guess?

I wouldn't have thought so, but nothing would surprise me.

| Assuming I (we) can recognise the maker of the two phones (that are in
| parallel), could I just measure the 'line' voltage actoss pins 2-5 and
| see if it's 12V as suspected?

Door entry systems aren't wired up like telephones. They usually have a
common wire, a microphone wire, and a speaker wire, to all handsets in
parallel, that connects to the speech unit, another common wire for the lock
release button, and a dedicated call wire from each flat's button on the
panel to the buzzer in the handset. It's not a standardised 2-wire interface
like a phone line. Because of that you have to have the right microphone and
earpiece speaker to suit the speech panel (some still use carbon
microphones) and the right buzzer to suit the voltage.

Owain