What is a polite way to ask "may I ask who is speaking?"
There are several ways to say it:

Polite: 

请问，你是哪位?

Qǐng wèn (May I ask), nǐ shì (you are) nǎ (which) wèi (identifier for people/position)

May I ask, who is this?



您好， 您找谁？

Nín hǎo (hello), nín (polite form of you) zhǎo (looking) sheí (who)?

Hello, whom are you looking for?



喂，请问您是谁？

Wei ("hi"- typical way people answer the phone), qǐng wèn nín shì sheí?

Hi, who is this please (..., 请问 is "please". 请问 ,... is "excuse me") ?



Impolite: 

喂？ 哪位？  

Wèi nǎ wèi  

Hi, who is this...?  



找谁呀？

Zhǎo sheí ya ("ya" is a participle equivalent to 啊 "ah" expressing surprise or doubt)?

Who are you lookin' for?

These are the ones I typically hear... although there are more.