Belgium central office telephone wire color code, residential quad:

  signal name   BE-vvt  BE-pet  USOC    info
  A  tip        white*  orange  green     0 VDC primary pair
  B  ring       blue*   blue    red     -48 VDC
  A2 tip        green   white   black     0 VDC secondary pair
  B2 ring       violet  gray    yellow  -48 VDC

  *The polarity convention of white/blue changed in the 1990's.
  USA: bell wire = yellow = usually connected to green

1. 1980's in-house wiring

  Each socket has 5 holes, 4 contacts with screw terminals and 1 key.
  There is one master socket with two extra screw terminals.  The
  in-house VVT cable connects all sockets in parallel.  The wire color
  code is:

    white a O   O s   violet
    blue  b O   O gnd green
             c d

  An external slave bell is connected over terminals a and s.

  The incoming star quad was connected to the master socket as
  the active primary pair : (-) to a, (+) to b,
  the reserve secondary   : (+) to c, (-) to d.
  Screw terminals c and d are insulated placeholders.  There is no
  terminal for the key hole.

  The telephone push button shorts a to gnd.  In old installations
  the drain wire of the incoming central office cable was connected
  to the in-house gnd wire.  Typically the wire was twined about the 
  conductor of the green wire in the screw terminal.

  The b wire is switched to the in-house bell wire s by the master
  telephone.  When off-hook the bells are disabled since dialing pulses
  would otherwise cause bells to tinkle.

  The master socket contains a bell enable switch, b to s, shunted
  by an RC circuit to guarantee low volume bell.  The test circuit,
  a to s, also gives old telephones some bell current.
  Schematically RTT86RG044:

    +--RC--+  anno 1988 test circuit 4.7 Kohm + 1 uF, or external bell.
    a     /s  
      /sw/ |  switch closed if no plug in socket. 
    b/     |  anno 1976-1985 shunt circuit: 4.7 Kohm/1W + 12 uF,
    +-C--R-+  guarantees low volume bell.

  Hence, unplugging the master enables the bells unconditionally.

  Telephone internal bell connection:
  old: b -- sw1 -- bell -- s, anno 1970 bells are in series,
  new: b -- sw1 -- s, enable slave bell,
       s -- sw2 -- bell -- DC block cap -- a, enable internal bell,
       bells are in parallel.
  When off-hook, sw1 disconnects s, and sw2 disconnects the internal

  A new telephone can be made slave by unbridging b -- sw1 -- s.
  Also, conversion for two wire operation can be done by rewiring
  the plug: disconnect s for a master phone, or connect s to b for a
  slave phone.

2. BE plug to rj11 socket adapters

  BE equipment, at least Ascom's up to 2000, has bell wire switching.
  If sold with a BE plug to rj14 socket adapter, the pinout of the
  adapter is:

    BE contact name  gnd   a   b   s      RTT92RG093 BE plug to rj14 socket
    wire color         g   r   bl  bk     RTT93RG5906 rj14 socket BE socket
    rj14 socket pin    2   3   4   5
    usoc color        bk   r   g   y
    signal name      acc   B   A   bell
    polarity         n/a   -   +   =p4    pin 5 switched to 4 by equipment

  This socket pinout and polarity matches USA practice.  Using a 2 wire
  flatcable gives 2 wire operation.  Adapters are cascadable if using
  straight through rj14 cable.  

  Answering and fax machines came with a BE plug to an rj25 socket
  adapter, on the side, and a BE socket on top.  The top socket was
  disconnected when the equipment went off-hook.  Older equipment used
  a 6p6c rj25 with pinout at the adapter:

    BE contact name  b'  gnd  a   b   s   a'   BE top socket a' b' gnd s
    wire color       wh   g   r   bl  bk  br   BE plug to rj25 socket fax
    rj25 socket pin   1   2   3   4   5   6   
    usoc color       wh  bk   r   g   y   bl   
    signal name      A'  acc  B   A  bel  B'
    polarity        =p4  n/a  -   +  =p4  =p3  switched by equipment

  Newer equipment used a 6p4c rj14 side socket as follows:

    BE contact name   a'   a   b   b'     BE top socket: a' b' gnd s
    wire color         g   r   bl  bk
    rj14 socket pin    2   3   4   5
    usoc color        bk   r   g   y
    signal name       B'   B   A   A'
    polarity         =p3   -   +   =p4    pin 2 and 5 switched by equipment

  The gnd and s are wired through from plug to socket.

3. House wiring for answering machines and facsimile equipment

  In larger houses telephone wiring typically consists of star wiring
  to some sockets, followed by parallel extentions further on.
  Recabling or rewiring is often infeasible due to the work required.

  Converting such an installation for office with fax required
  following steps:

  0. remove the RC bell circuit and socket switch,
  1. verify that all in-house cables are in parallel,
  2. connect the CO line to the in-house first pair, however such that
     the socket a is (+) and b is (-),
  3. connect the fax with an rj14 adapter to a socket,
  4. connect the house bell from gnd(+) to s(-),
  5. rewire telephone plugs to use the secondary gnd/s pair,
  5. near the fax leave a backup phone with plug wired to a/b.

  Using a BE plug to rj14 adapter lets the fax switch the a/b pair to
  the gnd/s pair.

  When a dedicated fax number is subscribed to upgrading is
  straightforward: swap the rj14 adapter for a 6p4c fax adapter, then
  connect the fax circuit to the a/b pair, and the voice circuit to the
  gnd/s pair.

4. 1990's dual circuit wiring

  Since BE93RG5302 the internal socket switch is no longer present.
  A socket thus is a 4 contact pass-through device which supports two
  independent pairs.  The new polarity convention is to connect (+)
  to a and (-) to b.  The in-house VVT wire color code to the socket
  contacts, case top view, is:

    white a+O   O-s   violet (b2)
    blue  b-O   O+gnd green  (a2)

  The case size is W5 x H8 cm.

  The active pair of the incoming star quad is connected to the left
  a(+) and b(-) terminals.  New BE plug to rj11 socket adapters are
  wired taking the a/b polarity into account:

    BE contact name    s   b   a   gnd
    BE polarity        -   -   +   +
    rj14 socket pin    2   3   4   5
    usoc color        bk   r   g   y
    rj14 signal name  A2   B   A   B2
    rj14 polarity      +   -   +   -  

  The polarity of the secondary pair is reversed, compared to rj14,
  but allows for backward compatibility with (2.) by using a rollover

  The secondary pair is typically used for ADSL.  For DSL a low-pass
  filter is required at the CO cable entry point.  At that point the
  incoming pair connects to the filter and to green/violet.  The
  filter output is plain old telephone service and connects to

  The green/violet pair forms a circuit to the modem installation
  point, without branches or further parallel extensions.  At the modem
  socket the two pairs may be split.  A fax adapter gives the primary
  at its rj14 socket, and requires the modem BE plug wired using the
  secondary.  A modem adapter gives the secondary at its rj11 socket
  and the primary a/b on its top BE socket.

5. The NTP2007 and TF2007

  The network termination point, NTP, is a socket installed at the
  CO cable entry point.  Its size is about 8 x 8 cm.  Three pair screw
  terminals connect to the 6 contacts of the socket.  Schematically,
  in top view with X a wire terminal:

      a2--RC--b2              TEL green
      10k + 470nF             a2+X white
      test circuit            b2-X blue
      IN black   in  O- tel   DSL blue
    orange a+X   +O  O+ O-    b1-X (violet)
    blue   b-X   -O     O+    a1+X (green)

  A double pole switch connects IN to TEL when no plug is inserted.
  The in-house VVT green/violet can be used as a DSL circuit to the
  modem location.

  The telephony filter, TF, is a 6 pin plug with an rj11 dsl socket.
  The a/b input signal is low pass filtered and relayed to the a2/b2
  tel output.  The plug's other outputs, the center column pins and the
  rj11 socket, are connected straight through from the a/b input.

  Inserting this plug in the NTP opens the switch and the filter thus
  passes telephone service from IN to TEL.  Also IN is connected
  straight through to DSL and to the plug's own rj11 socket.

6. CO cable characteristics:

  The primary pair runs to the left when facing the premesis from 
  the street.  The secondary pair usually runs to the right.

                    O/B intro  W/G
    headend feed     |  swg 22  |     headend loop
      -<- TP1 --<----+ junction +---->-- TP1 --+  swg 20
      -<- TP2 --<--------------------<-- TP2 --+  0.52 mm^2
                    street bundle

  RTT line characteristics
    on-hook voltage            42..50 VDC
    off-hook loop current max  80..84 mA (10 mA when no dialtone)
    impedance                  270 Ohm + (750 Ohm || 150 nF)
    dailtone                   440 Hz, -12 dBm, 0.2 Vrms

7. US rj11 4-wire color code residential
                             pin     1  2 3 4 5  6
  rj14 male plug hook underneath        b r g y
                       signal name     A2 B A B2
                          polarity      + - + -

  A/B = tip(+)/ring(-) = green/red  black/yellow

  CO marked pairs     1             2            3            4            5
  tip/ring = white/blue  white/orange  white/green  white/brown  white/slate

  Tip colors secondary  White (W), Red (R), Black (BK), Yellow (Y), Violet (V)
  Ring colors primary   Blue (BL), Orange (O), Green (G), Brown (BR), Slate (S)

Keyboard rj11 plug (view at keyboard side)
                             pin          1   2   3   4   5   6
  rj25 male plug hook underneath     shield +5V GND res DAT CLK
          reset is not connected                  cable