According to my dealership Denny Menholt Chevrolet as of 2018 the number hotline does not exist anymore service can only be done by a dealer. You will need to bring your vehicle to your manufacture dealership of your make say Chevy or GM or whatever. They will not charge any fees for them to give you the 4 digit master code. They only charge you if you want them to do it for you. For a Chevorlet 2001 Chevy Blazer the codes are 642 - 652 - 106010 (Chevrolet). Once you have all of this go to the service center and they will give you the code. Mine was 0150. Since their is no zero. just the last 3 numbers worked. Using the minute key to 50 and hour to 1 press am/fm button to confirm. Boom. Stereo now works.
