I think Casey did buy the phone from AT&T. If she stole the phone, she would not have any reason to mention it to LE. She had to mention the phone to LE because AT&T would have a record of her purchase. She also had to come up with a reason why she did not have the phone any longer.
First she told LE she left it at work on her desk. During her interview (7/16 4:11 at home) with LE, she was asked, "Okay, so the phone where you have the number saved was lost?" Casey replies, "Yes, I filed an incident report." (LE didn't ask if she filled out any reports - volunteering too much information?) She wanted LE to know the phone was missing and she did complete a report with Universal Security on the lost phone. She also said she made the report about 9 days ago.... July 7? maybe.
IMO, that phone was used to make calls Casey did not want LE to know about...Too bad Casey didn't realize, LE would be able to obtain a record of calls made on her account...new phone or old phone - the calls are documented. Or maybe it contains physical evidence.... i.e Caylee's blood.
I think the she pitched the phone out the car window... or maybe she buried it somewhere.