Configuring call waiting

I am using asterisk 1.8. In my chan_dahdi.conf file I have configured callwaiting=yes.

But I am not able use callwaiting feature. I am using IP phones. Should I make any other changes in config file to make it work?

chan_dahdi.conf has no effect on VoIP phones. Call waiting on VoIP phones is normally implemented within the phone itself without help from the PABX.