SIM900 : problem with some AT commands

Hello every one !!
I use the GSM module SIM900 for the first time. for more than a week i'm trying to send some AT Commands through the Hyper Terminal : Some works and some others just don't !!!
i have the SimCard and the antenna for the SIM900 :
the NETLIGHT led is blinking as to show that the SIM900 finds the network connection (64ms ON / 3000 ms OFF).
the Hyper Terminal is configured to : 115200 bps / one stop bit / no parity / 8 data bits.
(I'm using a USB adaptor and a RS232 cable to connect the SIM900 with PC.)

- when i send "AT" i get "OK"
- when i send "AT+IPR?" i get "+IPR:115200"
- ....
same thing for "AT+GMI", "ATE", "ATI", "ATZ"...it just work !!!

But when i use "AT+CPIN?", "AT+COPS?", "AT+COPN", "AT+CFUN=0", "ATDxxxxxxxx;".... i just get "error" !!!

anyone who could help ???

PS :
i also don't get "RDY" on the Hyper Terminal at the start but still can fix the baudrate or get the "OK" response when i send "AT" to the SIM900 !!!!
anyone who knows what does it mean ??

hello men for your question i hope to help you in because me too in your problem
let me tell you :
perheps we need somme softwars to enable the gsm to get ok response from the at commands ....you know ::::
or you can telephonne your operator of your gsm to fix this problem ....
when i type AT+COPS AND ENTER
WE RECIVE
AT+COPSERRO

TRY TO TYPE THIS :
AT+COPS? (AND HIT ENTER)
YOU WILL RECEIVE LIKE THIS :
+COPS 0.2.2"MOBILIS"
BY THE WAY THE NAME OF MY OPERATOR IS MOBILIS .
OR TYPE THIS :
AT+COPS=?
OR THIS :
AT+COPS=
WHEN YOU HAVE THE GOOD REPONCE SEND ME A MESSAGE IN MY PHONNE 00213798322488
MY PROBLEM IS WHEN I TYPE THE GOOD COMMAND LIKE :
AT+COPS?
I RECEIVE THIS : +COPS 0.2.2"" 5WITHOUT NAME OF OPERATOR ?
CAN YOU HELP FOR THAT ?
BEST REGARDS/ BEN AMMAR MOHAMED FROM ALGERIA . WHAITING FOR YOU !!!!!!

Hello !!
My problem was solved : I have changed the USART terminal and it worked !!!!
Try with "putty"
try other commands...and see what happens
try to change the SIM card... use another operator !!
Best Regards from TUNISIA