Someone correct me if i am wrong, but to my understanding backdoor number is that number that sometimes you get transfered to when you say the right things to the representatives or if you threaten to close your acc. You reach not automated service that allows you to discuss your situation with actually a human beeing, and my friend, it does work
I believe thats how they get the numbers, when they get transfered and the representative tells you in case the call get disconnected call this number!