NEW to Asterisk

Please Help.

Bottom line, I have Asterisk up and running on a MAC OS X version 10.4.2 with with 4 phones: 2 IP phones an Aastra 480i and Polycom Soundpoint 301 and 2 Xlite phones. I can call from one IP phone to the other IP phone, same goes for the Xlites, but I can’t call from one Xlite to an IP.
Does anyone have any tips on what I am doing wrong, I am desperate at this point so I could move forward in making out going calls other that to and from these phones.