T-Mobile US and AT&T have been trading shots over the prices AT&T charges for data roaming as part of the government’s investigation into a complaint filed by T-Mobile.

As we’ve previously reported, T-Mobile accused AT&T and Verizon Wireless of charging unreasonably high data roaming rates, making it difficult for smaller carriers to offer better deals to consumers. AT&T argued in a filing on November 14 that it “buys more data roaming than it sells both on a megabyte basis and on a dollar basis,” mostly through agreements with rural carriers, and that it pays more than T-Mobile does.

“For 2013 and 2014 AT&T’s roaming expense on a per megabyte basis exceeded that incurred by T- Mobile,” AT&T wrote to the Federal Communications Commission (FCC). “The average data roaming rate paid by AT&T in 2013 (42¢/MB) was more than the average data roaming rate paid by T-Mobile in 2013 (30¢/MB). For 2014, the average rate paid by AT&T through August (27¢/MB) is higher than T-Mobile’s projected average expense (18¢/MB). This is clear evidence that T-Mobile is paying commercially reasonable rates and that the relief requested by T-Mobile in its Petition should be rejected.”

T-Mobile accused AT&T of getting its facts wrong in a filing Friday. “AT&T mischaracterizes the facts of its negotiations with T-Mobile and the current status," T-Mobile wrote. "AT&T’s claim that it offered T-Mobile an LTE roaming rate of under $.18/MB is best described as illusory, since it is conditioned in such a way that T-Mobile would rarely, if ever, qualify for the offered rate… In fact, AT&T’s offered rate for LTE is 40 percent higher than the average rate that T-Mobile paid for data roaming across all technologies (including less efficient 2G and 3G technologies) in 2014 when excluding AT&T, and nearly 100 percent higher when considering the latest quarter. This is despite the fact that LTE technology is significantly more efficient than 2G/3G technologies, and that the cost to produce a MB of LTE roaming is therefore lower than for 2G/3G.”

The FCC should be skeptical of AT&T’s claim that it pays more for roaming than it receives, T-Mobile argued, writing that it sent AT&T more than 2,000 times the amount of data it received from AT&T. "More important, however, assuming that AT&T’s 'net payor' claim is accurate, it should wholeheartedly support adoption of the benchmarks T-Mobile has proposed, since it would stand to save the most on roaming costs,” T-Mobile wrote.

The FCC should also take into consideration AT&T's large size relative to other carriers, T-Mobile further wrote. “As a dominant firm with over 100 million subscribers, over 300 million covered POPs [points of presence] and market power to control prices, roaming is a far smaller fraction of AT&T’s costs than it is for other carriers," T-Mobile wrote. "By keeping roaming rates high, AT&T is able to drive up rivals’ costs much more than its own relative costs are affected. Further, carriers who roam on AT&T’s network (such as T-Mobile) are frequently forced to throttle and/or cap their customers’ data roaming on AT&T’s network to prevent economic losses, resulting in a distorted and inefficient traffic imbalance and carrier costs impacts.”

T-Mobile asked the FCC to provide industry-wide guidance on what constitutes reasonable charges for roaming. In 2013, T-Mobile says it paid far more in per-bit roaming charges than the big four carriers charge individual smartphone users. While per-bit roaming prices are going down each year, the amount of data used by customers is rising.

In a blog post last month, AT&T argued that T-Mobile “tries to blame its decision to throttle and/or cap data usage by its roaming customers on high data roaming rates. In doing so, T-Mobile conveniently ignores the fact that it has other options, including building out its own broadband network. A recent survey of FCC files indicates that T-Mobile has spectrum throughout the continental US, yet, as shown by the coverage viewer on T-Mobile’s website, T-Mobile has failed to build out its network in extensive areas throughout the Midwest, Mountain, and certain Eastern portions of the US.”

Verizon has sided with AT&T, arguing that the commission’s current framework of “voluntary negotiations backstopped by a complaint process is working as intended.”

Sprint has sided with T-Mobile, as has the Competitive Carriers Association, which advocates on behalf of carriers smaller than AT&T and Verizon Wireless.