Starcom and Zed Media, under the Publicis Groupe media and digital umbrella Vivaki, first won the consolidated business at the start of 2008 after a final shoot out with WPP's Mediaedge:cia.
At the time Starcom retained media buying duties while also adding the media planning brief and Zed Media, part of the ZenithOptimedia Group, retained online buying.
The telecoms giant becomes Maxus' largest client from 1 April 2010, with the account including all media planning and buying, excluding outdoor.
Matthew Dearden, BT’s marketing director, said: "We're delighted to be working with Maxus and Group M for our media strategy, planning and buying.
"This appointment will help us make the most of our media investment to build positive relationships with current and potential customers."