A man I don't know calls me and starts talking to me in Chinese. All is fine, except I don't know who he is. I sense I'm not the person he's looking for. How do I ask who he is without being too blunt: "你是誰?"
There are several ways to say it:
Qǐng wèn (May I ask), nǐ shì (you are) nǎ (which) wèi (identifier for people/position)
May I ask, who is this?
Nín hǎo (hello), nín (polite form of you) zhǎo (looking) sheí (who)?
Hello, whom are you looking for?
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") ?
Wèi nǎ wèi
Hi, who is this...?
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.
Alternatively, you could politely ask him who he is looking for:
Some people (at least in Taiwan) also say (你)哪里找, which to me doesn't seem either super polite or super impolite...really just depends on tone of voice.
(Side note: I'd also like to point out that
這是誰? is wrong, while
你是誰? is acceptable. You got this right, but it's not obvious to a lot of learners.)