"Hola, un minuto por favor."
Translation:Hello, one minute please.
Ditto, I have heard "Hello, a minute please." several times in my English speaking experience. Usually, it's in formal circumstances that people use "one minute" but both are correct according to the rules of English grammar. We are translating into English.
"Moment" is basically interchangeable with "minute" for the purposes of this idiom. There are also two distinct things this could mean, and i'm not sure which it is in Spanish. Is it, "Please wait a minute, and then I'll get to you" or "May I please have a minute of your time?"
The first and most likely reason is that "Hola" does not translate to "hey". The second reason is probably that is sounds weird since, "Hello, one minute please." is most likely said in a formal situation. You reach the reception desk and the receptionist says, "Hello, one minute please."