I tried getting Scottie to ring the bell each time we went outside. After a year, it didn't seem to be effective, because sometimes he'd just go play with it. He learned to hold his potty really well and doesn't indicate when he needs to go, so I always have to make sure I stick to his schedule.
Casie on the other hand will do a low grumble sound, kind of like Marg Simpson, but cuter.
I also have a porch potty for those miserable pouring days when I don't feel like getting drenched.