AT&T recently launched new unlimited plans, but it's also rolling out a "bonus" for customers on older Mobile Share Value plans. The catch is that those subscribers have to pay for that bonus.

AT&T is now contacting customers on Mobile Share Value plans to tell them that they're getting 15GB of extra data added to their allotments every month. That extra data isn't free, though, because the customers' plans are also increasing in price by $10 per month.

"We've added 15GB of data to your plan and increased your monthly rate by $10," an AT&T email obtained by The Verge reads. "Please sign in to review the changes to your account."

What's more, AT&T has confirmed that Mobile Share Value customers cannot opt out of the bonus data. The only way to avoid this added charge is to change to an entirely new plan.

"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 has said.

Folks who are still sticking with their Mobile Share Value plan are probably doing so because they're happy with what it includes and what it costs them every month. While I'm sure those people wouldn't mind getting an additional 15GB of data every month, they may not be so thrilled about also having to pay $10 more each month and having no way to opt out of that added data. 

AT&T launched new Unlimited Starter and Unlimited Extra plans last weekend, and so the carrier may be hoping that some Mobile Share Value customers will consider jumping to a new plan rather than keeping this price increase. The Unlimited Starter plan starts at $65 for a single line, while Unlimited Extra starts at $75 for a single line.