So AT&T is claiming that it can rollout 4G networks only if it is allowed to buy T Mobile (and combine the frequencies assigned to both). So what they really want is spectrum? As smartphones and tablets proliferate, so too have apps like one by Facebook that draw in ever-rising amounts of data. Monday’s letters cite AT&T’s contention that the T-Mobile deal will allow the carrier to expand its nascent 4G network to cover 97 percent of the country and an additional 55 million Americans. “The access aspect of this is so, so important,” Fred Humphries, Microsoft’s vice president for United States government affairs, said by telephone.