WASHINGTON, DC.- Until recently, visitors to Smithsonian museums on the National Mall in Washington, D.C., faced an unexpected challenge: they could not use their cell phones unless they were near a window or door. The solid construction of the Smithsonian’s historic buildings made it nearly impossible for cell phone signals to penetrate the walls. The 100-year-old National Museum of Natural History, for example, has granite-faced walls that are nearly three feet thick. The Smithsonian now has mobile broadband service in nine museums on the National Mall: the National Museum of American History, National Museum of Natural History, National Air and Space Museum, Hirshhorn Museum and Sculpture Garden, National Museum of the American Indian, National Museum of African Art, Freer Gallery of Art, Arthur M. Sackler Gallery and the Ripley Center (International Gallery). Visitors can use their mobile devices now for phone calls, Internet searches, podcasts about exhibits and tours