ah...that information alone (SIP PBX running) is very useful
So you already have a system running, then your task is almost as simple as what I said.
1) Get your carrier to install the E1 Termination (if they haven't already)
2) Obtain and install the Digium TE122 Card (or its PCI Express equiv depending on hardware) (make sure you set the jumper to E1 (comes as T1 by default)
3) Connect the E1 Ports together (30% chance you may need a E1 Xover cable depending on Carrier - but you could build one of these if necessary or buy one for safety)
4) Autodetect via the Elastix Hardware Detection GUI
5) It will now be available as the Dahdi/G0 Trunk
6) Set your outbound routes to use the new Trunk
7) test incoming calls - may have some further config depending on carrier, but can only test first.
Honestly, that is about it....
Further questions, please ask, but just don't be light on the detail...
Regards
Bob