This is interesting, but I see it as an example of social competition rather than conditioning. If I may hazard a guess: the sales team are all within earshot of each other, right?

People (and especially salespeople!) are competitive. When someone uses your software, how often the bell rings is a signal of that person's success -- a status symbol. Once one person starts using the software, everyone else will want to show off their success as well, so they adopt it as well.

My prediction: if the salespeople were separated, the bell would become an annoyance and they would disable it.

I wrote a piece of software for myself that does exactly what yours does, and it rings, exactly like yours does.