"Together, the city's and AT&T Wireless' engineering teams conducted joint testing and validation of the RF conditions taking place at one of their tower locations on East Ninth Street," the city said in a statement. "Both teams concluded that the AT&T 850MHz GSM cell site was causing significant interference to the City of Oakland's P25 System."

"The City of Oakland further advised the AT&T personnel that it had data proving many of their cell sites were exhibiting the same problem and advised them of the FCC rules giving absolute priority to public safety users and requesting they take immediate action to resolve this situation," the statement said.

AT&T confirmed it had partially disabled GSM service.

"AT&T is working closely with the City of Oakland to understand if the issues they've raised are connected in any way to AT&T's network," AT&T Wireless said in a brief statement. "In the interest of public safety and as a cautionary measure while we're looking into the matter, we have temporarily taken some 2G frequencies out of service at some cell sites in Oakland. We continue to operate on other 2G frequencies and our 3G and 4G service throughout the area is unaffected."

GSM is a second-generation cellular technology not capable of the fast data speeds that modern smartphones need. But while it is older technology, it supports customers using older and more basic handsets and helps fill in the gaps between high-speed coverage areas.