Revised (Ver. 2):
The information response "+CMGF: (0,1)" indicates that your phone supports both PDU mode and text mode.
It should work in Text mode as well.
Try checking your phone's functionality level using:
AT+CFUN=?
Set it to 1 for full functionality
Also check if requires PIN.
I am not very sure if these restrict changing the SMS mode, but try it once and let me know if it solves the problem.
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(Ver 1):
Error 303 means, the operation is not supported.
Try this first
AT+CMGF=?
This will tell you supported modes.
If the ans is (0,1) both the modes are supported.
I think your phone does not support text mode, U will have to work with PDU mode.
Hope this helps!!
Ankur Mundhra