It might be rudimentary, but I wouldn't say you're wrong.

Alternatively, pick up but answer the phone only with the word "Yes?", "Speak" or "You may proceed" (preceded by 'this line is now secure').

Then, when they ask "who is this?" answer that "if you don't know, you have the wrong number" and that "this call is currently being traced, pending review of a 'military tribunal'."

Do this with the flattest intonation you can manage.

That tends to get to them.