At one time I had a 2nd phone line at home, for my work router, and it was ex-directory, but it had a hand set so that Head Office could call me directly if need be. It also had an optional scrambler on it.................

I got VERY few calls on that handset of course, and I usually knew they were coming in advance, but on one occasion I got one of THOSE calls on it, goodness knows where they got the number from though. As I wasn't expecting the call I guessed it was one of 'those' ones and answered accordingly......
Me : Area 53, speak!
Caller: Er, I want to speak to Mr. Spackman please
Me : Pass code?
Caller : Er, I don't have one.....
Me (Sternly) : Do you know where this phone is connected to?
Caller : Er, no........................
Me : This is a secure line and should NEVER be called by unauthorised personnel! Do you understand?
Caller (squeakily) : Yes..............
Me : Forget you EVER called this number and delete it from your records, understand?
Caller : Yes...........
Plunk. Phone goes down.

That was the ONLY non-work call I ever got on that line,
