"A" vs "An" ....grammar trivia

[RICHARD]

Aha!

There's a answer to everything?

[Freedom]

FYI, we were taught to also use "an" for a word starting with an "h" which is silent.

So it would be an hour, not a hour.

But a house is correct, because you pronounce the 'h.'

[Taz_Zoee]

Just the other day I was sending an email to my co-workers saying I had an/a UBOC envelope. When using just letters which one would it be? Saying it outloud it sounds better to say "a" UBOC, but is that correct??