Kay-l-b
You wouldn't say May-belle. You'd say May-b-l. Same vowel sound.
I think you'll be correcting a long while yet! Sorry. Kay-lebbe is unusual.
I know (and now of) quite a few boys/men of that name and I've only ever heard it pronounced Kay-lebbe - never any other way. You can't just say that it's the same vowel sound as in Mabel, though, as clearly different letters can represent different sounds in different words. Even just staying with people's names, you'd never pronounce Isobel to rhyme with 'visible', would you?
As beautifully covered by Dave Gorman, would you pronounce Loughborough as Luff-ber-uff or Luh-ber-uh?
If anything, I'm guessing (prepared to be gainsaid by any native speakers) that, in the original Hebrew, it would have been the 'a' that was pronounced differently from the English-speaking-country standard and not the 'e' - so something like Kall-ebb.
On the subject of Hebrew/Bible names, I'm quite puzzled as to how the name Elishah (a man in the Old Testament and, AFAIK, the only well-known owner of the name for thousands of years) is now used pretty much exclusively as a name for girls (often spelt without the silent second 'h') - as an alternative spelling for Alicia. That's probably just me, though.