New To Asterisk

We are experimenting with Asterisk with the hope of putting it into production soon. I do have a small problem that has me somewhat baffled and would appreciate some guidance.

I have established the dial plan for internal and all appears to be fine. I can call any phone from any phone and access voice mail. When I take one phone outside the building, I can call it from inside but the “external” phone cannot call any of the other peers. I did set the server ip info up and the phone will log into the server.

Do I need a new dial plan? The phone is logging into the server so it is finding valid sip entries so how would I reference a different sip definition?