T-Mobile currently operates a roster of three agencies. Claydon Heeley Jones Mason holds the consumer direct marketing account, MSB+K handles business to business, and Bright operates on a project-by-project basis.
The incumbents have been asked to repitch alongside three unnamed agencies.
The move is part of an ongoing cost-cutting drive by T-Mobile and adds to speculation that the company is giving brand marketing a lower priority.
It follows last October's activity, when T-Mobile centralised its £27 million creative and media accounts - held by Bartle Bogle Hegarty and Starcom Motive - into Saatchi & Saatchi and Universal McCann.
BBH's grip on the account was jeopardised when T-Mobile's parent, Deutsche Telekom, decided to axe the One2One name and consolidate all its brands under the T-Mobile mantle. Universal McCann was already T-Mobile's global media agency, and Saatchis handled the work across the rest of Europe.
Saatchis' Frankfurt office was responsible for T-Mobile's much-maligned rebranding campaign starring Steffi Graf and Andre Agassi.
Earlier this month, T-Mobile axed its top-level UK marketing position, handing responsibility for marketing to its incoming managing director, Brian McBride.
McBride was most recently the vice-president, Northern Europe, at Dell Computers.
T-Mobile is the third-largest mobile network in the UK.