How to change the number of rings before voice mail picks up

My iPhone 12 goes to voice mail in 20 seconds. I have called ATT tech support twice to change that to 40 seconds, which I am told is the max. Both phone calls have taken about 45 minutes to both reach the right tech person and give them time to make the settings. I was told the first time that the change would take up to 24 to 48 hours. I waited and the calls still picked up after 20 seconds. On the second try I was told that the change was made and would be effective immediately. After I ended the call with the tech I tried the phone again several times and it still picked up after 20 seconds. Apple’s instruction is to call the carrier. In this case that is ATT. So I guess I will try once again, but I would love to hear from anyone else if they have resolved this issue successfully.