If someone intentionally blocks their number then *69 will tell them the last incoming number is unable to be traced. Do this:
1) Pick up your phone
2) Dial *77 and wait for a dial tone afterwards
3) Hang up the phone
4) Enjoy the inability of people who preface your phone number by punching in *67 trying to block their number from your Caller ID. They will get a recording that will, in essence, tell them that the person they are attempting to call has elected NOT to receive calls from anonymous origins and they are welcome to hang up and redial the number normally if they wish.
/story