After going seven years without a price increase, AT&T subscribers who are grandfathered into AT&T's unlimited data plan had to fork over an additional $5 per month starting last February. Now they'll have to do it again.

AT&T is raising the price of its unlimited data plans, which are no longer available to new customers, by $5 per month starting in March, according to DSL Reports.

The change increases monthly data rates to $40. Coming one year after the first price hike, it signals that AT&T intends to use regular price hikes to encourage legacy customers to switch to one of its current Mobile Share Advantage plans with data caps. There's also an unlimited plan introduced last year, but it's only for customers who also subscribe to DirecTV.

"Our Mobile Share Advantage plans and our AT&T Unlimited Plan provide several benefits that our legacy unlimited plan doesn't," AT&T told PCMag. "If you have a legacy unlimited data plan, you can keep it; however, beginning in March 2017, it will increase by $5 per month."

Unlimited, meanwhile, is not truly unlimited. Like many of its rivals, AT&T throttles users if they use more than 22GB of data in a billing cycle. That's a big enough cap that all but the most data-hungry customers won't notice it, but then again, those data fiends are precisely the type of customers who would want an unlimited plan.

AT&T's push to curb high data use comes as Verizon is doing the same thing. Verizon last week announced that people who average 200GB a month will be pushed off its unlimited tier. Verizon is offering those subscribers an ultimatum: move to a limited plan by Feb. 16, or get disconnected and switch to a different provider.