Shouldn’t the OP also learn how letters are sounded in the spanish alphabet (assuming he doesn’t already knows)?
Because if he says “A (sounded the english way) as in Argentina”, for instance, it might be quite confusing.
Since both him and the other person are speaking English, “éi as een aryhentina” works fine.
I thought of another good candidate for O: Orlando (it’s even pronounced almost the same way in both languages).
Wikipedia (inevitably) has an article on spelling alphabets, which includes the Spanish one.
Your customers very probably don’t know the Spanish spelling alphabet (*), but will be able to understand it
*) I for one dont’t know the German spelling alphabet (but the NATO alphabet which I had to learn for my marine VHF license). I guess much more Germans know the NATO alphabet that the German spelling alphabet, but we would readily understand someone speaking in the German spelling alphabet.
That wikipedia article is dog poo, I promise. Being kind, a pipe dream.
There is no such thing as a phonetic alphabet used by Spanish speakers all throughout La Hispanidad, and some of the words listed for that one are not good ones for Spanish, specifically for Spanish as pronounced by non-Spanish-speakers (they get mangled beyond recognition).