The aggressive unlimited price war between the top four US wireless carriers is the key culprit driving an overall decrease in the cost of wireless services for consumers, according to FierceWireless.

In April, the cost of wireless services declined 13% year-over-year (YoY), the largest percentage decline in 16 years, according to the US Labor Department. 

The sharp decline of wireless services revenue is symptomatic of the fierce competition that is upending the wireless industry. US wireless carriers are locked into an aggressive price war that's changing the way the mobile industry operates. Led by T-Mobile and Sprint, Verizon and AT&T have been forced to follow suit, implementing their own unlimited pricing schemes.

• AT&T added an unlimited plan for its DirecTV subscribers in January 2016. This gave the carrier’s DirecTV subscribers access to unlimited data streaming.
• In February 2017, Verizon, the largest telecom in the US, was the last of the big four telecoms to add an unlimited data plan. The company had previously stopped offering unlimited deals in 2011. 

• In an effort to maintain its mantle as the “Uncarrier,” T-Mobile has gone all-in on unlimited data plans. The company introduced the unlimited T-Mobile ONE in September 2016 as its only plan, stripping away all other data plans.
• Meanwhile, Sprint recently introduced a new unlimited plan that drastically undercuts the other carriers. For the next year, Sprint customers can get the first line of unlimited data for $50, the second for $40, and the remaining lines for free, Wave7 Research notes.

There's some concern that these competitive models might be unsustainable. US smartphone owners are consuming data at an exponential rate. By the end of 2017, the average data consumption in the US is likely to cross 6 GB per month, Chetan Sharma Consulting notes. At the same time, the average revenue per user (ARPU) for three of the top four carriers declined YoY. T-Mobile is the only US mobile operator that has managed to increase the revenue it squeezes from customers, with ARPU for postpaid phones growing 3% YoY. On the other hand, combined postpaid ARPU for Verizon, AT&T, and Sprint fell roughly 6% YoY, according to BI Intelligence estimates.