Что желаете? (from a waiter, receptionist)

someone is phoning me now. I'm saying -> Здравствуйте! Что вам угодно?
Sounds like you're already positioning yourself as a servant. Not even trying to ask who's calling.
I'd just say "Алло?" and wait for the explanations on who is calling and what they want.

If someone you don't know (not in your Contact List) is calling you on your cellphone it's already a WTF-situation so I would prefer that someone on the other side of the line explain themselves before saying something meaningful.