Phone conversation

When you are a caller

-- Hello?
-- 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