Our lil girl is also bell trained as well as potty pad trained.
We would walk her to the bell next to the door, take her paw, ring the bell with it, praise her then take her outside and praise her again. Whether or not she did her business didn't matter. We did this for almost 2 months. One day, she rang it 20 times...not a joke!
The funny part is, she thought that outside meant seeing people who wanted to play with her. She eventually realized you go outside to do your business and if you see people then that was a bonus.
When we are at work during the day, she goes on the potty pad!
It is amazing!!