How do Android phones turn mobile phones into empty numbers?

In standby mode, enter: **2 1*999999# and press the dial key ~ OK, and you're done. \x0d\ In this state, the phone number called by others is empty, so you can't receive the call. \x0d\ The code for canceling the empty status is: * 21. Input method is the same as above \x0d\ finally. X0d\ Set the mobile phone to an empty number \x0d "by entering" **2 1*999999# "in standby mode and then pressing the [Dial] key. After someone calls you, you will hear the hint of an empty number. Of course, you can't know the phone information. \x0d\ Release by entering "##2 1#" and pressing [Dial]. \x0d\ Principle explanation: Because it is sensitive, I checked some information. It turns out that this is a call forwarding service, that is to say, the call is forwarded to the empty number of "999999", and China Mobile does not charge for unsuccessful call forwarding, so it is still very useful. Similarly, you can also transfer to the number that has stopped and get the stop prompt tone. Of course, if you are brave enough, you can try to transfer your call to 1 10 (that is, "* * 21*/kloc-0 #"), 1 19, Peking University People's Hospital and other toll-free numbers. You can also try "**2 1* Jiao #" or "**2 1* Cecilia Cheung #" to express your condolences or make a phone call for predestination/love. \x0d\(* The following table is from China Mobile) \x0d\ Function setting and query cancellation \x0d\ Unconditional transfer **2 1* Phone number # # 21# * kloc-0/# \ x0d \ No signal transfer * * 6D. Unanswered transfer **6 1* phone number * 10* ringing time # # 61# * # # 61# * 61# \ x0d \ Transfer in case of busy * *