If you understand SIP, you should know that any SIP client needs to register to a SIP proxy to make it known to the proxy about its location (IP address, port etc.). Asterisk can act as a SIP proxy. I'm not sure about the exact "sip phone" you mentioned. But if your SIP phone is reachable over the LAN (i.e. has IP address), then you may configure it to register to the Asterisk server on the LAN. Exactly how to configure depends on how you would like to set it up, really.
How did you set up your Asterisk? If you compile and install manually, there is a chance for you to install a set of sample configuration with Asterisk. It would include sip.conf. For SIP, you just have to look into that file for SIP profiles and other related configuration (dtmfmode, jitter buffer, etc.). If your system is AsteriskNow or Trixbox based, they should be there out of the box.
By the way, Asterisk Forums :: Index
is likely a better place than here for Asterisk questions.