Not a bad effort!
Few things I would say, AT+CMGF, in your comments you state is "AT command to send SMS message", you may already know, but this actually sets the mode, PDU or TEXT AT+CMGF=1 sets TEXT mode.
I notice you do not issue a AT+CMGD, which will delete the SMS messages after you are done, a AT+CMGD=1,4 should do the trick, otherwise the SIM will get backed up with old messages and eventually reject them.
Perhaps of no interest to you, perhaps it is, but thought I would mention in case anyone else were to use your project
Also, are you sure that the 900A requires a lot of modification (you state in your report), I am no expert with these modules, and haven't looked into them in detail, but i'm sure the 900A is just a dual band version of the 900 (quad band)? What kind of modifications are required?