If ringing is received on the line, the CE/CT125 circuit
              (pin  22)  will be raised true.  If CD/108  circuit  was
              true,  the  modem goes OFF-HOOK.  If not the modem waits
              for  a true signal on CD/108 (pin 20),  before  it  goes
              OFF-HOOK.  The modem remains after 'off hook',  for 1.8-
              2.5 sec.  silent and then transmits the 2100 Hz.  answer
              tone  during  2.6-4.0  sec.  or till  100msec.  after  a
              response from the calling station. At least now 75 msec.
              silence  is  maintained.  The  2100 Hz  is  intended  to
              disable network echo cancellers or echo suppressors  and
              is reversed in phase every 425-475 msec. Now the circuit
              CC/107  (pin 6) is set true to indicate to the terminal,
              a connection is ready.  The calling modem reacts on  the
              answer tone also by setting circuit CC/107 (pin 6) true.
              A  response normally as mentioned is the frequency of  a
              binary  '1'.  If the equipment of the telephone  company
              does not have echo control devices, the 2100 Hz tone may
              be  replaced by any other tone e.g.  directly the binary
              '1'.  Only  after this sequence (or in ommission of  it)
              directly   the  autospeed  modems  will  determine   the
              baudrate norm.  In Europe,  with its V23-splitbaud  rate
              complexity, there are Hayes compatibles trying to select
              at the moment after the answer sequence to select:  V21,
              V23  (viditel),  V22 and V22bis.  This means actually  a
              detection  criteria for carrier frequencies at  980/1180
              (V21),  450/390  (V23) and 1200 V22(bis) modulated  with
              600 baud.  At the same time all kind of frequencies like
              2100, 550, 1800 Hz. and busy tone, dial tones around 425
              Hz. has to be ignored.