Skype, the internet voice-calling service, stepped up its efforts to gain a stronger foothold in the US on Monday, announcing free calls to landlines and mobile phones until the end of the year.

Skype, bought by the eBay auction site last September for up to $4.1bn, said all US and Canadian-based Skype customers could now make free calls to traditional phone numbers within their borders using its SkypeOut service.

The move undercuts Yahoo’s rival Phone Out service linked to its instant messenger program. Yahoo itself undercut Skype when it announced Phone Out for the US in March, which allowed users to call within the US and to more than 30 countries for 2 cents a minute or less.

Competition has become intense in the US, with Microsoft also introducing calling through its Windows Live Messenger program using Verizon’s Voice over Internet Protocol (Voip) service this month.

AOL has launched a service, and the instant messaging programs are competing with non-PC Voip services such as Vonage in offering cheaper phone calls routed over the internet.

Six months ago, Skype said only 15 per cent of its 66m users were based in the US. It passed 100m registered users this month, and Henry Gomez, general manager of Skype North America, said on Monday that the US proportion was growing.

“We think this will turbocharge growth even further ... Skype in the US is still relatively small, and we thought the time was right for doing this here,” he said.

At eBay’s analyst day earlier this month, it revealed that Skype’s penetration of internet users in the US was 4 per cent compared with 8 per cent in the UK, 13 per cent in China and 20 per cent in Finland.

Nearly 8 per cent of Skype users used Skype Out and Skype revenues were $32m, almost all coming from SkypeOut. Level 3 provides Skype’s Voip service, but Skype itself and its parent will be bearing all the subsidy for the free service.

Skype upgraded its service last week to offer direct SMS messaging along with the existing voice, text and video services.