Replaced battery: radio shows "LOC"

I have a 1999 Lumina. A cell in the battery started leaking acid so I replaced the battery yesterday. I bought the car new and the battery I replaced was the one that came with the car.
When I went to turn on the radio, the digital display showed "LOC." Reading the manual, I found this to be a normal condition when power is lost then re-applied and the "password" for re-activation of the radio must be provided. I've never put a password in the radio; what came with it when I bought it was some unknown set of numbers.
I retrieved the below procedure from the Net, which was supposed to allow me to, in effect, reset the radio on my own, without having to go to a dealer. ***************************************************************** straight from the how to section of this site:
1. Press and hold presets 2 & 3 for about 10 seconds until the first 3 digits of the display code are displayed.
2. Write the numbers down.
3. Press the AM/FM button, write down the next 3 numbers.
4. Call 1-800-537-5140, when asked, enter 202107, press the # button and enter the 6 digit code from the radio, you will get an unlock code, write it down.
5. Go back to the car, press the HR button and set the first 2 numbers, then press minute button and set the next two last numbers, press AM/FM button, SEC should be displayed, turn on radio.
To remove customers code, repeat step 5. you should see --- on radio face, the code is now out of the radio. *****************************************************************
This procedure has not proved to be 100%. I can get the data using steps 1 through 3 OK. Calling the number in step 4 will be answered but there is no request for the 202107. And, no request for the data I previously retrieved from the radio.
I am stuck: have I done something wrong by following this procedure? Have there been any changes to the above procedure? Is there another way I can reset my radio's code on my own without going through a dealer?
I think you in advance for your help.
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You have to go to GM dealer to unlock it. The service cost me $50.
Les ............................................................................ ..........................................

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According to my book the theftlock feature has to be activated before the loc feature works. Therefore someone must have activated it. check all your documentation for the code. I wrote a pointer to mine in the book...

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Try using the code 0000. Many times, people activate the lock feature but leave the default code set.

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submit the numbers you retrieved from your radio and I will send you the 4 digit unlock code.....

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