Load the version of SerialRelay from the web page in my sig link.
Once the modem has locked on to a signal, issue the command AT&F - please note that this restores the factory default settings.
Issue the command AT+CMGF=1 to put it into text mode.
Now issue the command AT+CNMI=2,2,0,0,0 which will send all sms / texts received to the Serial Monitor.
Now send a message to the device and it should repeat on the Serial Monitor. I have tried 26 characters and all appear.
+CMT: "+44xxxxxxxxxx","","14/04/06,19:26:40+04"Edit: Changed the reset code and added the AT+CMGF=1 line. It doesn't affect the functionality but the result wouldn't be as shown.