This is Ken.
Asking who is on the telephone:
Excuse me, who is this?
Can I ask who is calling, please?
Asking for Someone:
Can I have extension 321? (extensions are internal numbers at a company)
Could I speak to…? (Can I – more informal / May I – more formal)
Is Jack in? (informal idiom meaning: Is Jack in the office?
I’ll put you through (put through – phrasal verb meaning ‘connect’)
Can you hold the line? Can you hold on a moment?
How to reply when someone is not available:
I’m afraid … is not available at the moment
The line is busy… (when the extension requested is being used)
Mr Jackson isn’t in… Mr Jackson is out at the moment…
Taking a Message:
Could (Can, May) I take a message?
Could (Can, May) I tell him who is calling?
Would you like to leave a message?