How to create trunk between asterisk and sip server

hi everyone,

i am using sip server that functions as registrar and proxy, now i have to integerate asterisk with it, call should be made via asterisk .
user get registered with sip server and call should be routed via asterisk to pstn and other sip phones.
asterisk has been configured to call any extension but now the sip phone has to be registered with sip server not asterisk.
please help me with this.