Change the DN to something that's not a DID, and change keys from SCR to SCN. They'll still be able to see the key ringing, but won't hear it (I mean who stares at their phone all day?). Alternatively, you could change KEY 00 to KEY 06 or higher so those appearances will be visible show up on the screen when you hit the 'shift' key on the 3904, and change the LHK setting to the same value as the first line appearance on the phone. For example... if KEY 06 is set to SCN 1234 - make LHK to 6. Key's 0-5 will show up as either blank or whatever else you want to program them as.
Or go ahead, make KEY 00 SCN 1234 - and so long as you're running later firmware on the phone (usually off a PBX that has CS 1000 v6 or higher on it), you can change the key label on the phone to whatever you want it so the person sitting at the phone can't say "yeah call me at x1234). I think... we do this to change the 'in calls' KEY 00 on 3905's from the CDN to the position ID. I don't have a 3904 here to try that on for a non-ACD phone.