If I fancy a "bit of a Lark" my standard response when I get one of these calls is:-
Hello, you have reached the Ministry of Defence south west area command centre. your telephone number has been automatically noted for future reference, and you may receive follow on calls, in the meantime please have the name of the agent you wish to speak to, together with your security clearance code available .
almost invariably the call is terminated, and in most cases is not repeated.!! I'm thinking of having this put as an alternative message on my Ansaphone.