The website also told me that 6 was not available for me. My modem went bad and AT&T insisted on a truck roll to confirm before replacing. When the tech arrive, I asked him is I could get 6 and he said he'd look. He went to the DSLAM, did some tests and said I could. He then called and had the line provisioned for 6 which works great for me. I think maybe you should ask one of the techs to call in and you shouldn't have a problem.
