IBM MOBILE COMBO ADAPTER PRO/100 SP - Manual (Page 31)

To modify the value of an S-register:
TYPE: ATSn = r
where n is the register number, and r is the new register value.
PRESS: Enter
S0 Auto Answer Ring Number
This register determines the number of rings the modem will count before
automatically answering a call. Enter 0 (zero) if you do not want the modem to
automatically answer at all. When disabled, the modem can only answer with
an ATA command.
Range: 0-255
Default: 0
Units: rings
S1 Ring Counter
This register, Ring Counter, is read only. The value of S1 is incremented with
each ring. If no rings occur over a six-second interval, this register is cleared.
Range: 0-255
Default: 0
Units: rings
S2 AT Escape Character (user defined)
This register determines the ASCII valued used for an escape sequence. The
default is the + character. The escape sequence allows the modem to exit data
mode and enter command mode when on-line. Values greater than 127 disable
the escape sequence.
Range: 0-255
Default: 43
Units: ASCII
Appendix B. Modem function B-5