You can't do anything. They're confused themselves and don't know what they really want yet.
Send mixed signals too. If they don't catch up and stop with the mixed signals after a while, ignore their mixed signals altogether.
I would back off a little bit and wait for them. A more confident person might approach them and tell them how you feel.
you ask them straight up what the heck do they mean
It depends. If they are using flags, then I set their flags on fire. If they are using smoke, I douse it with water. If they are stammering around trying to say something, I set their hair on fire.
Don't answer the phone.
shrug it off. it might be a seductive tactic. they might need admirers that do or give them what they want. if it's someone really tantalizing and I recognize what they're doing, i play dumb when they're being suggestive.