AT&T is adding $10 to the monthly bills of customers with certain grandfathered mobile-data plans and not letting them switch back to their older packages. AT&T is pitching the change as a "bonus" because it's also adding 15GB to the customers' monthly data allotments.

"Enjoy more data," AT&T says in a support document. "Starting with your October 2019 bill, you'll get an additional 15GB of data on your Mobile Share plan. This bonus data comes with a $10 price increase."

Paying an extra $10 for another 15GB isn't a bad deal as far as US wireless prices go, but that's only true if you actually need the extra data. The plans getting the data-and-price increases already had between 20GB and 60GB of data per month at prices that ranged from $100 to $225. Now those plans have 35GB to 75GB and cost $110 to $235. (The data allotments can be shared among multiple people on the same family plan.)

These Mobile Share Value plans were introduced in December 2013 and are apparently no longer offered to new customers. This is at least the second time this year that AT&T has added $10 and extra data to customer bills; a previous increase took effect between March and May and mostly affected a different set of Mobile Share Value plans, according to another AT&T support document.

No way to opt out

AT&T confirmed that there's no way to opt out of the new $10 increase, The Verge reported yesterday.

"We are communicating with some customers regarding changes to their mobile plans. Customers have the choice to change their plan at any time and can always contact us with questions or to understand their options," AT&T told the news site. But as The Verge noted, the option to change plans doesn't include going back to the older, cheaper ones.

The benefits for AT&T are clear. The company gets another $10 a month from customers and probably won't actually have to supply the extra data in most cases. If customers needed more data than they already had, they probably would have changed to different plans already.

The other benefit for AT&T is that customers hit with the price increase may decide to switch to one of AT&T's newer plans, some of which have data limits and some of which are "unlimited." If you're switching to a new plan, make sure to check out the fine print—even the plans advertised as unlimited have plenty of limits.