Could you get people to use the NATO phonetic alphabet for the spelling part? I suppose a challenge is that many people don't know the whole thing, even if they're aware it exists.

NATO phonetic is to be understandable over a noisy radio channel, if you want just distinct sounds then Talon Voice users settled on shorter ones easier to use all the time:

    air a
    bat b
    cap c
    drum d
    each e
    fine f
    gust g
    harp h
    sit i
    jury j
    crunch k
    look l
    made m
    near n
    odd o
    pit p
    quench q
    red r
    sun s
    trap t
    urge u
    vest v
    whale w
    plex x
    yank y
    zip z

Interesting, that some words don't start with the letter they represent.

“Plex” ?