Modems have 41 registers, designated S0 through S95.  These can have their values changed by 
commands.  If you use a command to change a register value, the command remains in effect 
until you turn off or reset your modem.  Your modem then reverts to the operating 
characteristics specified in its non-volatile memory.  Refer above for information on how to 
use the AT commands to manipulate the “S” registers.

Register  Function                                              Range/Units
S0        Auto-answer Ring                                      0-255/rings
S1        Ring counter                                          0-255/rings
S2        Escape code character                                 0-127/ASCII
S3        Carriage return character                             0-127/ASCII
S4        Line feed character                                   0-127/ASCII
S5        Backspace character                                   0-32, 127/ASCII
S6        Dial tone wait time                                   2-255/seconds
S7        Remote carrier wait time                              1-255/seconds
S8        Comma pause time                                      0-255/seconds
S9        Carrier detect response time                          1-255/0.1 second
S10       Carrier loss time                                     1-255/0.1 second
S11       Touch-tone dialling speed                             50-255/milliseconds
S12       Escape character guard time                           0-255/0.2 second
S13       Reserved
S14       Echo, response, dialling, originate/answer            Bit-mapped register
S15       Reserved
S16       Modem tests                                           Bit-mapped register
S17       Reserved
S18       Length of modem tests                                 0-255/seconds
S19-20    Reserved
S21       CTS, DTR, DCD, DSR and Long Space Disconnect          Bit-mapped register
S22       Speaker and response                                  Bit-mapped register
S23       Remote digital loopback request, data rate and parity Bit-mapped register
S24       Sleep mode timer                                      0-255/seconds
S25       Data terminal ready delay                             0-255/0.01 second
S26       RTS and CTS delay interval                            0-255/0.01 second
S27       Asynchronous/Bell/CCITT modes                         Bit-mapped register
S28       Reserved
S29       Flash time                                            0-255/0.01 second
S30       Disconnect timer                                      0-255/10 seconds
S31       Autoscan, EC result code                              Bit-mapped register
S32       XON character                                         0-255/ASCII
S33       XOFF character                                        0-255/ASCII
S34-35    Reserved
S36       LAPM failure options                                  Bit-mapped register
S37       Connection speed                                      Bit-mapped register
S38       Hang-up delay                                         0-255/seconds
S39       Flow control options                                  Bit-mapped register
S40       MNP options, break types                              Bit-mapped register
S41       Compression, retrain, DCE flow control, block modes   Bit-mapped register
S42-45    Reserved
S46       Data compression control                              136 or 138
S47       Reserved
S48       V.42 negotiation options                              0, 7 or 128
S49-81    Reserved
S82       Break options                                         3, 7 or 128
S83-85    Reserved
S86       Connection failure code                               0, 4, 5, 9, 12, 13, 14
S87-90    Reserved
S91       Transmit attenuation                                  0-15/dBm
S92       Fax attenuation                                       0-15/dBm
S95       Extended result code options                          Bit-mapped register