Hi
The gsm modem is conexant. Its very good, i experimented with it using hyper terminal and used most of its functionality before i designed my PCB.
Rob / Andy : The modem works in 2 modes, either text mode or binary mode, the default mode is text mode, I experimented with that and issued the command AT+CMGF=1<CR><LF> from the 8052 and to my surprise i got the response OK, that means it accepted this command.
When i issue the initialization command AT+CNMI=2,2,0,0,0<CR><LF> i get the command echoed back with error message 305 which means invalid text mode as the manual explains. No further information about this is available in the manual.
I sent email to my distributer and will wait to see what tech support say.

Andy Neil wrote:
It is possible that the clock rate & divisor you're using on your 8052 gives a slightly greater baud rate error than the PC's COM port, and that everything just happens to conspire against you in this particular instance, so that the modem rejects your command.
This might be true because Hyper terminal is more tolerant to baud rate errors than the uart, i sat the baud rate to 14400 and the hyper terminal to 19200 and i managed to see the data displayed on the screen in older project.
My error margin is 2 small and I think it should have no problem with baud rates. I will double check.
Maybe your 8052 system puts some noise on the line, or your MAX232 is just slightly out of spec?
if this was the case all commands would be rejected and not some.