Here's a way to nudge customers into higher price plans. Sprint-owned Virgin Mobile will lower the price of its Beyond Talk "unlimited" voice, data, and text messaging plan to $55 per month, but raise the cost of its two cheaper Beyond Talk plans to $35 and $45 per month, respectively.

The new prices take effect next Tuesday, July 19, but unfortunately for consumers, the definition of "unlimited" will change in October. At that point, Virgin will start throttling its 3G data speeds when usage exceeds 2.5GB per month—likely to about 256 kbps or below, a spokeswoman said. Virgin did the same in February for its once-unlimited Broadband2Go Internet plans.

Virgin claims only 3 percent of its current customers use more than 2.5GB of data a month.

"After reaching this level, this minority of customers may experience slower page loads, file downloads and streaming media. When a customer's next month begins, the data usage meter starts back at zero with unlimited 3G speeds," the company said in a statement.

As for its Beyond Talk plans, the unlimited messaging/data and 300 minutes plan will set you back $35 each month instead of the previous $25 per month, while the unlimited messaging/data plus 1,200 minutes will get bumped from $40 to $45 per month.

Existing Beyond Talk customers can keep their plans as long as they don't let their accounts expire. So if you're thinking of switching to the contract-less mobile ISP, do it now.

"We are all facing the same situation and this is the best way for Virgin Mobile to maintain the best network experience as data usage explodes," David Trimble, vice president of Virgin Mobile USA, said in a statement. "Our no-contract and postpaid competitors like Cricket, MetroPCS and T-Mobile have either implemented more stringent constraints and/or don't disclose slower speeds and data caps. We believe this adjustment – with no hard cap or overage charges for more usage – gives the most value to the largest group of consumers. It's important to Sprint and the Virgin brand that we be as up-front as possible with our customers."

Earlier this year, T-Mobile introduced "unlimited" plans that throttle data after 2GB. Following in the footsteps of AT&T, Verizon last week also ditched its unlimited data plans in favor of several tiered options: $30 for 2GB per month, $50 for 5GB, and $80 for 10GB.

Virgin Mobile, meanwhile, will also start selling the Motorola Triumph, Motorola's first Android phone for Virgin, on July 20.

For more from Sara, follow her on Twitter @sarapyin.