Who's calling, please?

• You have just called a company and asked: could you put me through to Mr. Smith, please?
• The receptionist will ask you: 'Who's calling, please?'
• He or she wants to know who you are.
• Calling = telephoning.
• Who's = who is.
• Who's calling? means who is telephoning? / who are you?
• You can say this at work or at home, it doesn't matter.

This sentence is an example of telephones, business.