AT&T on Thursday said it would slow uploads and downloads for 3G (HSPA+) users who consume more than 3GB of data in a month. This means that users of 4G LTE phones from AT&T with an an unlimited data plan will probably see slower data speeds when they consume more than 5GB of data in a month.

Last June, AT&T implemented a policy of slowing its unlimited data users' speeds only when they fell into the top 5% of users in terms of consumption. Previously, AT&T didn't slow speeds for unlimited data users.

The policy change will not affect users who have AT&T service and are on one of the company's tiered data plans.

AT&T said on its website the changes are all a response to "soaring mobile broadband usage and the limited availability of wireless spectrum."

The company said its seen a 20,000% increase in data consumption by its users over the last five years, and has gone from 7 million smartphones on its network in 2006 to 39.4 million in 2011.

AT&T's biggest rival, Verizon Wireless, also slows down service speeds for its top 5 percent of data users, but only in areas where the network is crowded. And unlike AT&T, once its customers leave the congested area their speeds return to normal, according to the company.