This is Jim Smith calling from A1 Computers, in London. Could I speak to Mr. Jones, please?

Notes:

• You have called a company and the receptionist asks: 'who's calling, please?'
• That is: who are you?
• You say: 'this is (Jim Smith).' (My name is Jim Smith.)
• …calling from = I am calling / telephoning from (company name) in (name of your city).
• Could I speak to ... please? 'Could I...please?' is the polite way of asking for something.

This sentence is an example of telephones, business, requests.