asterisk users - May 2005 - Integrating Asterisk into our Legacy PBX <--Newb

I have been successful in setting up asterisk and making workstation to
workstation SIP calls. But I am lost when it comes to anything past that.

We are trying to integrate this asterisk server into with our Executone
(432?) PBX to allow us to make outbound SIP calls between our disparate
locations.

We have a T1 card in our PBX, and the Digium TE110P card in the Asterisk. We
have the T1 card connected to a CSU and the CSU going into the TE110P. And
the Asterisk server connected to the WAN.

We get dial tone from the T1 card but that's as far as I get! Where do I
begin?

Thanks in advance!!