How to configure group call

im completely new to this…can anyone tell me how to configure group call in asterisk

i tried this
exten => 105,n,Dial(SIP/104&SIP/102&SIP/101)

i able to call all the three sip phone but at a time only one can communicate

plz tell me how to configure it

thank you

Do you mean you would like to page the phones, or you would like to create a conference with the phones so you can ring them all, and automatically put them in a conference and everyone can speak?