Its not! Its a scam!
http://www.nbc-2.com/story/22811096/some-att-customers-falling-victim-to-scam
http://wafflesatnoon.com/2013/01/28/scam-warning-att550-com-phishing-scam/
And of interest, I also found this one:
http://www.philly.com/philly/blogs/montco_mem ... -ATT-bills.html