The appointment will come as a blow to the incumbent, WDMP, which has held the £4 million consolidated account for the past two years.
Carphone Warehouse held informal talks with several undisclosed below-the-line shops, before awarding the business to Kitcatt Nohr.
The agency will work on a project-by-project basis for the company's broadband and telephone services, and will create launch campaigns.
WDMP was hired to handle direct marketing for Carphone Warehouse's fixed-line brand TalkTalk in 2004. In 2005, it was given the consolidated direct marketing brief.
It has since produced campaigns like a "Fashion Picker" mailer, positioning the phones as vital fashion accessories.
Gavin Wheeler, the chief executive of WDMP, said: "It doesn't affect anything that we're doing. It's a rapidly expanding business, so there are always new areas that come up which would be too much for one agency to handle."
The win follows the hire of Richard Madden, the former Claydon Heeley chief executive, who joined as the planning director in July just two weeks after the creative director Phil Keevil.