AT&T is allowing new and existing customers on shared plans to take advantage of double the data, but the window of opportunity to sign up on the promotion shuts a month after opening.

The promotion doubles 15 GB of data to 30 GB for two lines, which keeps its $130 price tag. It provisions three- and four-line accounts to 40 GB, five and six lines to 60 GB, groups of seven and eight to 80 GB and accounts with nine and 10 devices to 100 GB.

All that's required of customers is to proactively sign up for the promotion. The monthly charges remain the same, according to David Christopher, chief marketing officer for AT&T Mobility, who says businesses can also take advantage of the promotion.

"We're now making our simple and flexible plans a greater value by doubling the data for the same price," said Christopher. "Families and small businesses can share a huge bucket of data on the nation's most reliable 4G LTE network, which now covers more than 300 million people. With these new double data plans, and smartphones for $0 down with AT&T Next, there's never been a more affordable time to be with AT&T."

So long as customers sign up for the double data promotion by Oct. 31, they can hold onto the larger allotment of data indefinitely. However, accounts enjoying double data after the promotion periods will be brought back down to earth if account holders make any changes to them.

For a couple dining on a 30-GB supply of data each month, AT&T says the two can surf the Internet for 600 hours. They can stream 40 hours of HD video, stream 250 hours of music, upload 1,000 photos, download 200 apps and send 1,000 emails bearing attachments.

Sweetening the deal and making AT&T's case that consumers should get an iPhone 6 from the wireless carrier, the company is offering a $100 credit for customers who activate a new line of service for a smartphone.

With the window to trade-in iPhones to AT&T just about shut (it closes Sept. 30), the $100 credit is a bit of a consolation for those who missed out on trading in their old handset for a $300 credit.

T-Mobile is still unwilling to disclose when it's iPhone trade-in promotion ends, though it is still promising to match whatever deals its competitors come up with and to tack a $50 credit on top of it. Verizon's trade-in promotion lasts until Oct. 15 and Sprint maintains a solid buyback program that follows market demand.

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