IMEI provided is not a valid AT&T IMEI

I've been trying to get my used iphone 4 unlocked to no avail.  I was first told that it was on another att account (it isn't)  and now I have been told repeatedly that it is not a valid ATT IMEI.  I am very frustrated at this point.  The phone is 5 years old and there's NO reason why it cannot be unlocked other than the fact that ATT does NOT want to do it.

I was on the phone for 3 hours the other day--the first customer service person told me to call Apple and have them unlock it--I did it knowing that they would not and they didn't,  and they told me to call ATT back.

The 2nd person seemed more knowledgeable and actually talked to someone else about the situation who decided my current provider would have to unlock it.  I called them and they laughed and said that they NEVER unlock phones and only ATT could do it.

The 3rd ATT person refused to give me to a supervisor and instead referred my case to someone else so at least I finally have a case number.  A few hours later I was told that it is not a valid ATT IMEI yet again.  If that is true, then why does my phone show ATT in the settings and not allow me to change them to get data with another provider?

I'm at a loss and tired of being on the phone with a different person every time.  Please ATT, unlock my old phone so I can have some data.