I have a little bell on my Commuter but not on my MTB. I don't use it if I can avoid it, because people tend to either turn around, give me a scared look and then jump in any direction if they hear it or feel offended. Calling out, as many of you describe, would be considered rude and I guess it wouldn't work anyway - nobody is used to it. So I usually just approach them slowly and pass if there's enough space somewhere.
We have a lot of mixed bike/foot-paths and they are real hazzle as a cyclist.