but responding to SMS is not working in my program.
This is quite simple providing that your modem is capable to hand text format (if you have only PDU format then you need to convert text to PDU first)

Code:
AT+CMGS=+39xxxxxxxxxxxx,13
where +39 = international prefix for Italy (you will change as needed)
xxxxxxxxxxxx = phone number you will send the sms.

Immediatly after having sent this AT command you go listening the modem :

answer should be: ">"

When you receive this character then you sent your text (max 160 chars) terminating with 26

HSEROUT ["This is my test",26] and you have sent your message.

If you don't receive ">" within 5 seconds you must abort the command sending: HSEROUT [27] and repeat the command.

Hope it will help you.

Al.