If you are new to voice and Avaya especially, I would start looking up information/admin guides on Communication Manager. Being that it is a G3 you will have an older software load, so certain 'newer' features may not be in your system but most of the screens will correspond to the manual.
Use the Display command instead of the change command until you are more comfortable with the system. Gives you better control - no accidentally saving a form with the wrong settings. Don't type reset system 4 during production.
1. Do a list trunk-groups as suggested above. Then go and map out all the lines coming in so you have a grasp physically where they are, and on top of that you can start figuring out who the providers are and if needed, Circuit ID numbers.
2. Now contact the providers of the circuits and lines you have. They should be able to give you a list of all numbers terminating to that trunk.