When I first started with Lucy I would respond when I saw her by the door. So, she'd go and sit, and I'd walk over, make her ring the bell, and then let her out. I didn't start ignoring the plain sitting until she was reliably ringing the bell to go out. Right now Mocha still thinks that sitting by the door works. You need to recognize that response right now, but make him follow through with the bell BEFORE the door opens. Am I making sense? Right now he doesn't get that ringing the bell opens the door. You have to make that association by being REALLY consistent with the bell. It may take a while so stay patient. It could take a month or more from what I've heard others say.
Good luck!