Being that I'm consistently slow, I get passed ALLLLLL the time. By far, MOST of the time the rider says "good morning" or "on your left" right as his/her shoulder comes even with mine. Close enough to startle me. I'd much rather have a little more warning.

So - today, I came up on a roadie and passed him. When I was several feet still behind him but gaining, I called out - "Just letting you know I'm back here!" He looked back, smiled, raised a hand in acknowledgement.

And - before you get excited thinking Who hoo, she's got it going on, she passed someone for the first time! Didn't quite go that way, as soon as I passed him, he started drafting off me! He stayed right on my wheel w/out a word for about 3 miles then HE passed ME like I was standing still -

No obvious effort on his part either. But - when HE passed me, he didn't say anything until he was totally passed me then called back "How far are you going?"

Bell or voice? I dunno - find whatever works for you, just give SOME warning, whatever you decide.