The agency, which was already Nextel's lead agency, has been competing against the Sprint roster agency Publicis & Hal Riney for the account.
The latter will now produce business-to-business ads, while TBWA will handle everything else.
Spend for the launch, which is expected by the end of summer, has not yet been determined. However, according to data from Taylor Nelson Sofres Media, the two companies combined spent more than $1 billion in 2004.
Sprint is America's third-largest telecoms company with a market share of 11.8 per cent, while Nextel is the fifth-largest, with an 8.9 per cent share.