Trying to get help on the phone

I have called AT&T customer service several times to try to get an answer to a question about my landline bill after I had looked it up online and saw something I didn't understand and was concerned about.  Three times I was told that the person didn't have access to their "tools" to look up my account but a supervisor would call me back that day (one person said the supervisor would call back in less than an hour, and he even gave me a ticket number.)  I don't understand why a "supervisor" had to be the person to call me back, but in any case, I never got the return call in any of those three instances.  A fourth person told me that what I saw online that I was asking about was incorrect, and oh, by the way, would I like to purchase their internet and TV services?   Why is it so difficult for AT&T to answer a simple question from their landline customers?