AT&T is raising the price of its grandfathered unlimited data plans by $5 a month, the second such increase in the past year. The price increase affects longtime mobile customers who have held onto unlimited data plans for years after AT&T stopped selling them to new subscribers.

The latest price increase was reported by DSLReports yesterday, and AT&T confirmed the move to Ars.

"If you have a legacy unlimited data plan, you can keep it; however, beginning in March 2017, it will increase by $5 per month," AT&T said. The unlimited data price had been $30 a month for seven years, until AT&T raised it to $35 in February 2016. The price increase this year will bring it up to $40. That amount is just for data: Including voice and texting, the smartphone plans cost around $90 a month.

AT&T encouraged customers to move to one of its new plans, most of which have data limits, saying the newer packages "provide several benefits that our legacy unlimited plan doesn’t." For example, the newer plans support mobile hotspot connections allowing a phone's Internet service to be shared with another device. AT&T had stopped selling unlimited smartphone data to new customers and to customers who are switching plans, but last year introduced a new unlimited plan that's available only to people who also subscribe to DirecTV or U-verse TV.

While Verizon Wireless is forcing customers to give up unlimited data plans if they use more than 200GB a month, AT&T has used several other tactics to limit the usefulness of the grandfathered plans. In 2011, AT&T began revoking unlimited data plans from customers who jailbreak iPhones to use unauthorized tethering services. AT&T also used to throttle unlimited data customers for the remainder of the month when they used more than 3GB or 5GB, depending on what device they had. AT&T has made the throttling less punitive, now applying it after customers use at least 22GB in a month, and even then only when customers connect to a congested cell tower. Government agency attempts to punish AT&T for unlimited data throttling have so far come up empty.