Of all the carriers you could possibly get your iPhone 6 through, AT&T is one of the worst. But starting on Sunday, signing a new two-year contract with AT&T is going to get a little more attractive, especially if you use a lot of data: for a limited time, Ma Bell is offering double the data for Mobile Share Value Plans. Beginning Sunday, Sept. 28, AT&T is offering new promotional Mobile Share Value plans that double the amount of data offered on plans between 15GB to 50 GB of data.