Not able to send sms using AT commands !

Hi,
I am using GM28 sony ericsson modem. I need to send the SMS using
AT commands.
I have done following settings :

AT+IPR=115200 <baud rate>
AT+CMGF=1 <message in text format>

this is what i get :

AT+CMGS=<phone number> <ret>
> <message> <ctrl + z>
ERROR: 500 <unknown error>

I have been able to send the sms sometimes using the same settings. But
it is unreliable and doesnt work most of the times.
I am able to make phone calls, receive phone calls and also able to
receive the SMS, the only problem is with sending the message.

Could anyone help me with this ?

Thank you

--chuck