Complex sentence: Bell and the other carriers in the area were especially supportive of an overlay; they did not want to force their customers to have to change their numbers for the second time in less than a decade or force "en masse" reprogramming of cell phones.
Split into simpler sentences: Bell and the other carriers in the area were especially supportive of an overlay. Not only would it have forced customers to change their numbers for the second time in less than a decade, but it would have forced "en masse" reprogramming of cell phones.