Discuss PGroot's answer to: How to make it so when you call someone they can't see what the number is?

In the US and Canada, *67 before the number blocks your number from being shown on Caller ID, except for emergency services (like 911 or police) or toll free numbers (like 800 area code).

However, some people have anonymous call rejection, so you have to leave your number unblocked to call them.