There are a large number of different LTE bands in use around the world and seven reside in the 700 MHz to 800 MHz frequency area. North American carrier AT&T own licences to operate in two LTE bands around this frequency point - band 12 and band 17. Today's news is that in a network status update report delivered to the FCC, Federal Communications Commission, AT&T revealed that it has turned on band 12 700 MHz spectrum within its network. However, it also reported that it is also experiencing operational difficulties in meshing the various bands at the 700 MHz point that AT&T operate (band 12 and band 17). The US carrier is working with several smaller carriers (believed to include T-Mobile US and U.S. Cellular) for technical assistance in creating roaming between the various frequencies used.
AT&T has something of a chequered history when it comes to the interoperability of its 700 MHz LTE bands. Following AT&T being granted access to band 17 LTE, the carrier stated that it would not be able to support bands 12 and 17 simultaneously as it would be too expensive and cause too much interference. Two years ago in September 2013, AT&T changed its mind and agreed to support band 12 and band 17 side by side, developing and integrating new technologies (multi-frequency band indicators, or MFBI) into its network to support interoperability. Over the summer, AT&T confirmed that it had installed the MFBI technology into its wider network as well as one of their small cell vendors, but the second small cell vendor had not completed testing. The business had until the end of September in order to get its MFBI up to specification. AT&T's discussions, the business said this regarding its discussions with other carriers concerning meshing the two bands together: "[we have continued discussions] with several Band 12 operators to stay aligned technically to support LTE roaming but are still encountering multiple issues associated with support of voice until the operators are fully deployed for VoLTE because of a lack of support for Circuit Switch Fallback capability. We are investigating the ability to support data only roaming but additional details need to be analyzed."
When it comes to device support, AT&T seems to already have this covered: "many of our devices have been sold with Band 12 filters and can be upgraded via software to support Band 12. We will begin the process of transitioning to Band 12 late in 2015 and / or early 2016." Band 12 is already in use by T-Mobile US and US Cellular (and this is why it seems almost certain that AT&T has been working with these carriers). T-Mobile US, especially, has made something of a selling point of offering band 12 as the low frequency provides for decent coverage, which may be supplemented by mid- and high-frequency masts in areas of network congestion. From AT&T's statement, it has made material progress in the last two years but has slipped from the FCC deadline. However, for those customers with an AT&T device that supports band 12, it appears that at some point in the next six months, your device will gain a software update providing support for the new band.