Never mind on the model, it's an SX-200 Generic 217. When you dial '9' you'll see trunk cards light up. My guess is someone did you a favor and changed either the type of signalling or polarity on the trunks. It's not exactly a DIY fix unless you have a set of manuals and about an hour to read up on trunking, class of service, and features. You could be landing an open trunk, or just be restricted from dialing.

Carl