It might happen for different reasons that you don’t want your number to show up as caller when you call someone. To hide caller’s number, iPhone made it easy for us. You may need to disable “Caller’s ID” by going to Settings > Phone > Show My Called ID > Turn Off.
When you turn it off, your caller ID (aka your phone number) won’t appear for the person who is receiving your call. To turn it On again, you will need to go to the same option and turn it ON.