For disable V90,X2, & V42+ Modem

MODEM MODEL	Disable X2	Disable V.90	Disable V.90 & X2	Disable V.42
LT Win modem		"-V90=0"	"-v90=0s38=0"
EM6800U - Aztech External Modem (Serial Port)	AT32=34	ATS32=66	AT&U16
UM9800 - Aztech USB V90 External Modem			AT+MS=34,1,33600,33600
USR Sporster	S32=34	S32=66	S32=98	S15=128
USR Courier	S58=1	S58=32	S58=33	S27=32

Intialization strings enable and disable Lucent / V90 modems

K56Flex only (V90 disabled)	S109=0
K56Flex or V90	S109=1
V90 only (K56Flex disabled)	S109=2

Intialisation strings enable and disable Rockwell / V90 modems

V90 preferred (K56Flex will still work)	+MS=12,1
V90 only (K56Flex and V34 disabled)	+MS=12,1,34667,56000
K56Flex preferred (V90 will still work)	+MS=56,1
K56Flex only (V90 and V34 disabled)	+MS=56,1,34000,56000
V34 (V90 and K56Flex disabled)	+MS=v34

For V90 Rockwell base chipset modems disconnection problem plse try this init string :
at+a8e=,,,0&w

For K56 flex disconnection problem, try this
+MS=56,0,300,46000,1,0S91=13S11=50%e0

Bring the modem setting down from 56kbps to 33.6kbps
For Rockwell base
at+ms=11

For Lucent Base (LT modem)
ats38=0

Enable the k56flex but slowly increase the speed
For Rockwell base
+ms=56,0,9600,34000,1,0\N3%C3 - limit to 34000
+ms=56,0,9600,36000,1,0\N3%C3 - limit to 36000
+ms=56,0,9600,38000,1,0\N3%C3 - limit to 38000
+ms=56,0,9600,40000,1,0\N3%C3 - limit to 40000
+ms=56,0,9600,42000,1,0\N3%C3 - limit to 42000
+ms=56,0,9600,44000,1,0\N3%C3 - limit to 44000
+ms=56,0,9600,46000,1,0\N3%C3 - limit to 46000
+ms=56,0,9600,48000,1,0\N3%C3 - limit to 48000
+ms=56,0,9600,50000,1,0\N3%C3 - limit to 50000