AT&T has a new unlimited wireless data plan, but only for those who also subscribe to DirecTV or U-Verse.

Beginning Tuesday, new and existing subscribers can spend endless hours surfing the Web, watching videos, and playing games on their mobile device.

The new AT&T Unlimited Plan includes limitless data, talk, and text for $100 a month. Additional handsets cost $40 each month, while a fourth gadget can be added for free. It's available for a limited time to wireless users; business customers are not eligible.

AT&T stopped offering unlimited plans to new customers in 2010, but existing users were allowed to keep them. In February, however, those users will see a $5-per-month increase—from $30 to $35.

All "unlimited" AT&T plans, however, are not truly unlimited. Those who use 22GB of data per billing cycle may face slow speeds "during periods of network congestion."

For added incentive, DirecTV and U-Verse TV subscribers not on the AT&T wireless network can, for a limited time, make the switch for $500 in credits; you'll need to make an eligible trade-in or buy a new smartphone via AT&T's Next plan.

Similarly, wireless customers looking to join the DirecTV fray can add a TV package starting at $19.99 per month for 12 months, with a 24-month agreement.

Combine your AT&T wireless and DirecTV/U-Verse bills to save an additional $10 per month.

"It's the perfect reward for our valued customers who like to take advantage of our integrated offers of TV and wireless services," AT&T CEO Ralph de la Vega said in a statement. "Video traffic continues to grow on our network as fast as ever because people enjoy viewing their favorite video content on their favorite devices."

AT&T promised a wide range of new video entertainment options in 2016, but did not elaborate.

"We've got the content consumers want, we're delivering it to the devices they want, and we're making it as easy as ever for them to consume as much as they want," de la Vega said. "And, they will get a high-quality video streaming experience from the start. No compromises in video quality."

AT&T completed its acquisition of DirecTV last year.