If it is a straight T1, the Magix won't have the DNIS digits anywhere in the programming. It just takes whatever the service provider is sending and routes it accordingly (if they are sending 100, it will ring extension 100).
If it's a PRI, you could look under PRI, Dial Plan Routing, and it might be listed as one of the tables. Again, it could just be that the service provider is sending digits that match an extension/extensions on your dialplan (xxx-xxx-4369 would send the call to 4369 automatically if they were sending 4369).