Toshiba V.90 - Manual (Page 39)

AT Commands
AT command formats
20
2 = No answer, busy, etc.
3 = Training failure V.90, 56K or V.34
4 = Protocol failure if required by \N4, for example.
21 For PCM connection only, a hexidecimal 6-bit pattern of T1
frames with robbed-bit signaling.
22 For PCM connection only, the downstream digital loss.
Result Codes:
Ln Monitor speaker volume
This command sets speaker volume to low, medium, or high.
Result Codes:
Mn Monitor speaker mode
This command turns the speaker on or off.
As described n=0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11
ERROR Otherwise
L0 Low volume
L1 Low volume (Same as L0)
L2 Medium volume (default)
L3 High volume
OK n=0,1,2,3
ERROR Otherwise
M0 The speaker is off.
M1 The speaker is on until the modem detects the carrier
signal (default).