Casual contraction of 请问您是哪位. Polite enough thanks to 您 and 位, but more clipped, used after the opening 喂 ('hello') when you want to keep it brief. Common in Beijing speech.
grammar
Drops 请问 ('may I ask'), 是 ('are'), and the question particle. Restoring all of it: (请问)您(是)哪位(?). The honorific 您 alone is what keeps this short form polite.
