UK Telephone System

Come a major event, the one thing that may keep going is the phone system. Well at least for a while.
It’s dead easy to splice into the UK telephone system in an emergency. 

Note:- It is illegal to do this BUT we are talking EMERGENCIES here.

Between you and an exchange a telephone essentially runs off two wires.

The ‘A’ wire which sits at Zero Volts to Earth
The ‘B’ wire which sits at -50 volts DC to Earth
The voltage comes from batteries at the exchange.

Pin /Wire/Volts
2/B/-50
5/A/0V
2/B/-50
5/A/0V
2/B/-50
5/A/0V
2/B/-50
5/A/0V
Type
Pair
One
Two
Three
Four
No.12 One
Orange
White
No.10 Two
Orange
White
Green
Black
No.10B Two
Orange
White
Green
Black
55M Four
Orange
White
Green
Black
Red
Grey
Blue
Brown

You need to connect a handset across a pair of these wires i.e.

Orange/White or Green/Black or Red/Grey or Blue/Brown.

A handset is made out of a cheap wall mounted phone, a BT to RJ11 adapter, and two miniature crocodile clips Black and Red
Looking at the standard BT plug, it’s got 6 pins (but usually only 4 are used).
You need to use pin 2 and 5 with pin 1 the opposite side to the locking tab.
Cutting the RJ11 plug off, you’ll be faced with any of these popular colour codes. Connect as shown. 
That way you’ll still have a serviceable phone should you enter a building that has a phone line but no phone.
Pin Number
2
3
4
5
You Need To Use

B
Ring
Earth
A
Black Clip
Red Clip
Old Wiring
Y
G
B
R
YELLOW
RED
New Wiring
K
R
G
Y
BLACK
YELLOW
US Wiring
R

G
RED
GREEN
Alt Wiring
K
Y
BLACK
YELLOW

Fit the clips as detailed and in turn, connect across the pairs or drop wire. 
I say in turn because you will hear the dial tone when you’ve got the correct pair but don’t keep hold of the wires with your fingers. If someone calls, you’ll get a shock of about 75 volts AC. 
That stings a bit and if you are up a pole, it might be enough to cause you to lose grip.

Once connected, you can use the phone as normal BUT incoming calls won’t “Ring”.
That’s because the master socket in the house filters the ringing tone onto a different pin i.e. Pin 3 of your BT plug.
There again you’d only be interested in calling someone for help not receiving calls.

As for OPSEC. The home owner (if present) may hear a ping or two as you play about and if they pick up the phone when you are talking, they will hear you and I’m pretty certain they will want to say something to you.
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