Pretend to be your own butler.
Answer simply "Yes?" (Longish drawn-out, slightly-rising inflection)
When the caller asks for "Is that Mr Bloggs?", reply "I will see if he is available, Sir. Is he expecting your call? Who may I say is calling?"
Then leave the phone for a longish time, (possibly playing a tape of echoing footsteps down a long marbled hall), and return with:
"I'm sorry, Sir. He is not to be disturbed at present. May I suggest that you call 01xxx yyyyyy to book an appointment with his accountant/stockbroker/building services manager/dietician(*) to discuss the matter?"
(*delete as appropriate - or substitute similar professional consultant suitable for the particular caller.)