Conference Problem

Hi. All,
What kind of protocol does the conference function in sip-communicator base on? I tried to connect the client with an Asterisk conference server, we set a conference number for calling,but when I dialed,it kept reminding me of inputting the pin number, and I entered the pin number in the number pad, but it didn't work.
Would you please tell me why this happened and how I could use it for conference through a conference server. Our server is based on SIP.
Thanks for help
 ;
Best Regards.
Demi Xu