Hi Gil
Thank you for your help (again). Of cause, it is easy to control the chip to be a modem and I am trying to do so. My problem begin when I use the chip to call another one. I don't know how the dialer should know that the call has been accepted. So I just let the accepter to send an acknowledgement over the DTMF mode. As the result, the system can work but not very good. Sometimes, it worked but sometimes it didn't. So I think I don't know about the exact procedure used to write and read via DTMF mode.

Narin