WASHINGTON — For nearly three in 10 households, don’t even bother trying to call them on a landline phone. They either only have a cell phone or seldom if ever take calls on their traditional phone.

The federal figures, released today, showed that reliance on cells is continuing to rise at the expense of wired telephones. In the second half of last year, 16 percent of households only had cell phones, while 13 percent also had landlines but got all or nearly all their calls on their cells.

The number of wireless-only households grew by 2 percent since the first half of last year. Underscoring the rapid growth, in early 2004 just 5 percent had only cell phones.

Households with cell phones who rarely if ever use their landlines grew by 1 percent since the first half of last year.

Such families often either have their landline hooked exclusively to a computer or rely so heavily on their cells that they ignore landline calls because they are probably from telephone solicitors, said Stephen Blumberg, senior scientist at the Centers for Disease Control and Prevention and an author of the report.

The trends have an important impact on polling organizations, which rely chiefly on calls to random landline phone numbers. Calling cell phone users can be more costly for pollsters, in part because federal law forbids unsolicited calls to cell phones made by computerized dialing systems used heavily by pollsters.

Studies have shown that so far, people who have only cell phones don’t give significantly different answers to questions than those who use landlines. Pollsters, though, are under growing pressure to survey the growing number of cell phone users and some already do so.

Also affected are the telephone industry and emergency service providers, who can find it harder to locate people calling from a cell phone.