Belgium central office telephone wire color code (residential)
  signal name   BE      BE-pet  US      info
  A tip         blue    blue    green     0VDC
  B ring        white   orange  red     -48VDC
  A2 tip        green   white   black
  B2 ring       violet  gray    yellow  US: usually connected to green
  gnd (old)     green                   push button shunts B to ground
  bell (old)    violet                  connect slave bell from violet to white

  RTT 4-pin telephone jack layout (Belgium specific socket)
    a O   O s
    b O   O gnd

  CO trunk to socket connection
    B white  a  /s    violet B2
    A blue   b/  gnd  green  A2

    a--RC--s  10k + 470nF test circuit or external bell
    b--RC--s  4k7 + 12uF shorted if no plug in jack, master phone bell

  The blue white pair runs to the left when facing the premesis from 
  the street.  The green violet pair usually runs to the right.

    headend feed    BW  swg 22  GV    headend loop
      -<- TP1 --<---/  junction  \--->-- TP1 ->-+ swg 20
      -<- TP2 ----- street bundle ------ TP2 -<-+ 0.52 mm^2

  RTT plug to rj11 jack adapter
    color CO trunk     g   w  bl   v
    plug name        gnd   a   b   s
    rj11 jack color    g   r  bl  bk
    rj11 jack pin      2   3   4   5
    signal name       A2   B   A  B2
    polarity           +   -   +   -
    rj11 US color     bk   r   g   y

  RTT line characteristics
    on-hook voltage            48..50 VDC
    off-hook loop current max  80..84 mA (10 mA when no dialtone)
    impedance                  600 Ohm, 270 Ohm + (750 Ohm || 150 nF)
    dailtone                   440 Hz, -12 dBm, 0.2 Vrms

US rj11 4-wire color code
                             pin     1  2 3 4 5  6
  rj11 male plug hook underneath     -  b r g y  -

                               pin   1  2 3 4 5  6
  rj11 female jack hook down color   -  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
  rj11 male plug hook underneath     shield +5V GND res DAT CLK
          reset is not connected                  cable