My usual response to any call of that nature (once I am given the opportunity to speak) is to ask the following, using a puzzled voice: "Computer? What computer?"
On a couple of occasions I have just stated, using an officious voice: "Please state your security clearance code." and have then left the handset, "off-hook", with the transmitter muted.
Leaving the handset "off hook" results in a situation that was known as a PG (permanent glow) and that would result in the automagical application of the graduated howler to the circuit. Eventually that, too, would time-out and the circuit would then be "parked" so as not to monopolise any of the exchange equipment.