A leading US bank has discovered that energy-efficient windows in its newer buildings are blocking mobile phone signals.

Bank of America now faces paying premium access charges to wireless carriers to enhance indoor mobile phone signals, according to its senior vice president for strategic planning and technical architecture, Eileen Bridges. She was speaking at Mobile & Wireless Enterprise 2008.

With more than 15 buildings in Charlotte, North Carolina, where the bank has its headquarters, the three buildings designated as "green" are the ones where the signal problem has been detected, Bridges said.

Bank of America is making good progress on a multi-year deployment of voice-over-IP phones for nearly all of its 200,000 workers, but the mobile phone problem in the green buildings wasn't anticipated, she said. And Bridges' staff isn't yet sure how widespread the problem might be. However, she suspects "we're at the tip of the iceberg."

Several analysts and IT managers at the conference said they had never heard of the problem before, but Bridges said the interference has been linked to a special doping material used in the manufacturing process.

Metal is a well-known enemy of mobile signals, and companies in some large steel-framed buildings know that they need to enhance signals, especially deep in those buildings' interiors. An entire industry has developed around Fixed Mobile Convergence to allow companies to link wireless phones to wired network architectures, sometimes by using phones that operate on both Wi-Fi and mobile networks.

But metal in window materials is a more recent development. In recent years, some architects have relied on new windows with a thin metallic coating that reduces energy usage by reflecting heat into the building in the winter and out in the summer.

3Com announced a Prestige series of glass in 2006 that eliminates the metal, partly to reduce corrosion and partly to reduce the cellular interference.

On the flip side, some businesses have used the transparent metal linings in some window glass as a security advantage, blocking Wi-Fi piggybacking from outside – not to mention hackers sitting in a parking lot hoping to read data moving inside the building. Astic Signals Defenses sells such glass specifically for that purpose.

Bridges said having wireless carriers add in-building equipment to boost mobile phone signals for Bank of America has been complicated. Sometimes the booster equipment will reach only to certain floors of a building, leaving some end users without access. That, in turn, has led to employees wondering why they weren't favoured by the IT shop to get acceptable signals, she added. "It's involved," she admitted.