Is it a DIT or a DID? dit could be a line fault, are you dialling internally or externally? Is the station in make busy Cmd E50 should be 1 for in service. Is this a fault or a new install? Have you tried reseating the card? Is the led on on the card? When 3779 rings, can you answer it? Is it a qmaster linked system (may be worth unmonitoring then remonitoring the line)? If it is a Qmaster site has there been a number swap? delete the station in lines and let the Qmaster relearn it on next use, Heck try this even if there hasn't been a number swap, it won't do any harm, but at least check it is registered as a digital.
Well thats a few to be going on with.
Let us know how you get on and/or what the solution is!