We also had to add our own doorbell. It came as part of a motion-sensor and alarm system for anything within range. I feel very comfortable leaving wife at a CG at night with the system armed and ready. ANY intrusion is immediately noted and then she can act accordingly (we do carry, but for the most part the alarm system is all one needs as a deterent).
The system is very accurate (in fact, sometimes TOO accurate), but is also extremely loud and decisive. We can be way in the back and hear it, even with the AC's or engine running.
Did I mention it also needs to be walked several times a day, and eats a lot of dogfood? He's an 85lb German Shepherd Dog, very people-oriented and friendly but he doesn't look it.