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
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: