Dial plan help

Need a liitle help setting up a dial plan.

This is what I am trying to do:

I am connected to a PBX with T1 tie lines, the SIP phone’s I have are assigned extensions in the Asterisk that match with extensions in my PBX,so calling from my pbx to the SIP phone’s work fine. What I want to be able to do is assign extensions in the Asterisk to different applications that are not associated to a SIP phone and be able to dial it from my PBX. I have been able to create these within the Asterisk and call them from a SIP phone, but when I call from my PBX I get the recording from Asterisk stating that “the number dialed is out of service”.
I am new to Asterisk so this is probably not as complicated as I have tried to make it.

Thanks for any help