I used to ring my bell on the MUPs when passing others, until they decided to assume I was going to pass on the right and cause a potential issue. Now I just call out "On your left" or "Passing on the left, stay straight". I always slow down to pass with care just in case, but calling out has been more successful to me than the bell.

In my experience you've got to time it right, if you call out On your left too soon than when they don't see you pass they'll assume you aren't moving to the left side of the path, on the other hand if you wait too late then they'll move out of being startled.