Over one in four US adults now live in households with cell phones or smartphones only, and that number can only rise.

Back in 2010 only 26% of US adults didn't have a home phone number but, according to the latest instalment of GfK MRI's Survey of the American Consumer, that number has jumped by 70% over the past four years.

Today, 44% of US adults don't have a landline phone installed at home, and this growing trend is strongest with Millennials. Within this age group, which GfK MRI defines as those born between 1977 and 1994, only 36% report having a landline, way ahead of Generation X (born between 1965 and 1976), 45% of whom make do with either a smartphone or cell phone only. Perhaps unsurprisingly, the oldest generation, those whose births predate the Baby Boomers, are the most likely to preserve their landline -- only 13% of this group have cut the cord and gone mobile.

The GfK MRI study, which polls the opinions and product consumption habits of 25,000 US adults, notes that the figures, although part of a growing trend, are also affected by how cable, smartphone and internet services are packaged and sold.

For instance, in the US Northwest, bundling of services, including a landline, is network providers' most common approach and as such is the geographical region where landline ownership is still high -- 63% of adults in this region live in a home with a landline and only 28% of Northwest respondents said that they don't have or use a landline telephone.

The study was published in the same week that the Pew Research Center released its latest study into US consumers' online habits. It found that for 7% of Americans, their smartphone is their only means of accessing the internet. Dividing that figure by age reveals that 15% of 18-to-29-year-olds are ‘smartphone reliant' when it comes to the web.

GfK's study pegs cell phone ownership in the US at 93% of adults while younger people are most likely to have a smartphone. Pew puts the figure at 64% of adults (up from 35% in Spring 2011) and their popularity is highest among younger adults. GfK's data shows that 88% of Millennials own smartphones, followed by 79% of Generation X, 56% of Baby Boomers and 20% of Pre-Boomers.