AT&T said its 700 MHz systems are now essentially fully interoperable with those of some smaller carriers, making good on a vow it made to the FCC more than three years ago. And that’s likely good news for T-Mobile and some other operators.

The nation’s second-largest wireless network operator said in September 2013 that it would begin to support 700 MHz interoperability and would begin to offer Band-12 capable devices, marking a surprising about-face from its previous claims that such a move would be costly and unnecessary. AT&T followed that up in March 2015 when it disclosed in an FCC filing that it was working with several smaller carriers to follow through on its commitment to support interoperability between Band Class 17 and Band Class 12 in the 700 MHz band.

AT&T has consistently declined to name those smaller carriers, but they likely include T-Mobile and U.S. Cellular, perhaps among others. In its progress report with the FCC, AT&T said it had met the goal of supporting interoperability in the lower band of 700 MHz airwaves.

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“In the intervening year since the Fourth Progress Report, there is near universal availability of Band 12 devices for smaller carriers and a plethora of new devices have been introduced,” according to AT&T’s filing. “In addition, carriers have continued to roll out Voice over LTE service on 700 MHz A-Block Spectrum.”

AT&T also said that it had completed deployment of multi-frequency band indicator (MFBI) software on its 700 MHz airwaves, which enables its network to operate simultaneously in both Band 12 and Band 17, supporting devices in both band classes. The operator now supports roaming on compatible LTE networks and plans to enable VoLTE roaming “for carriers with an incompatible 3G network” during the first half of this year.

A lack of interoperability has plagued the industry for years. Lower 700 MHz A Block licensees long argued that vendors like Apple made equipment for AT&T's Band Class 17 and Verizon Wireless' Band Class 13, but not for those smaller companies such as C Spire Wireless and U.S. Cellular that hold 700 MHz A Block spectrum in Band Class 12. The FCC auctioned 700 MHz spectrum in several different bands in 2008.

The announcement is good news for AT&T customers who can now roam onto the 700 MHz networks of T-Mobile, U.S. Cellular and perhaps some other smaller operators more easily. It’s also good news for those smaller carriers because it gives them an increased opportunity to monetize their roaming agreements with AT&T.

The news is particularly noteworthy for T-Mobile, which in recent years has been purchasing 700 MHz Band Class 12 spectrum, and is using those licenses to expand its network into new geographic locations.