Built in modem not reply to incoming calls

In my recently bought Inspiron 9400, (with Windows Media Center ) the built in modem does not reply incoming calls. I have call back privilege on my company server to access the network; it calls correctly, the phone rings in reply but id does not get the line.

I configured the connection the same way (without any special parametrization) I did with my Latitude D500 (with WXP PRO) and Dimension 4550 (with WXP Home) both of them working correctly in replying the same type of incoming calls.

Is any initialization string necessary? Or which steps can I do to check the modem capability to reply an incoming call?

Thanks in advance

Giampaolo