1. Is your modem communicating with a PC at the specified speed? You seem to use 2400 baud rate...

2. Do the trick with a second serial port, two resistors (experiment from 10K down to 1K which ever works) to pick the Rx and Tx lines an monitor on a PC terminal. You will see if the commands are received and what the modem answers.

3. Post the results to check here.

Ioannis