When you are a caller
-- Is that John?
-- Can I speak to John?
-- May I speak to John?
When you are a receiver
-- This is John speaking, who's that?
-- Who's speaking please?
-- Who's calling please?
When you ask the caller to wait
-- One moment please.
-- Just a minute please.
-- Hold on please.
-- Hang on please
When you ask the caller to leave messages
-- Can you leave the message?
-- Can I take your message?
-- Would you like to leave a message?
-- Would you like him/her to call you back?
When calling the wrong number
-- I'm afraid you got the wrong number
-- Sorry, I think you reach the wrong number
When asking the receiver to transfer the line
-- Can I have extension 1234 please?
-- Could you please transfer the line to Mr. John please?
When finish talking
-- Nice talking to you
-- Talk to you later