More American households are ditching their old telephones: 4 out of 10 only use cellphones, a government survey shows. That's twice the rate from just five years ago, although the pace of dumping landlines seems to have slowed down in recent years. The CDC has been tracking phone use for a decade, and the number of households only using cellphones had been rising by about 5 percentage points each year. Lately, the increases have been smaller and last year it only went up 3 percentage points to 41 percent of U.S. homes. Why the slight leveling off? The lead researcher speculated that people are holding onto their landlines because it is part of their Internet and cable TV package, or that we're hitting a ceiling for those people willing to completely abandon landlines.