How do I allow my SIP phones to use the landline

I have setup an Asterisk server and I want my sip phone devices to be able to not only communicate with themselves but also be able to make landline calls. I have noticed that in many business environments the SIP phones are set to where you would dial a certain number or certain numbers in order for the PBX to connect you to the landline. How do I make asterisk connect the SIP phones to the landline in order to make landline calls from the SIP phone. … eConn.html