WPP and Denstu Aegis Network look to be the front-runners in the race for Vodafone's estimated £400m global media planning and buying account.
WPP’s Wavemaker is the incumbent on the business, while Carat is leading the challenge for Dentsu Aegis Network.
Publicis Media is also pitching.
It is understood that Omnicom is no longer in the running after being involved at an earlier stage of the review.
Vodafone declined to comment and none of the agencies would comment.
The British telecoms giant's periodic review follows its decision to bring its biddable media, which includes search and social, in-house a year ago.
Industry observers are taking a close interest in Vodafone as a test case because a growing number of brands are in-housing some marketing services and reducing the traditional scope of agencies.
Sara Martins de Oliveira, global director of brand and media at Vodafone, told Campaign in March that she expects the company to in-source more media over time.
"Within five years, I think most of media will be biddable," she said. "It means there is no need for middle people. You can do it yourself if you’re data-rich."
Vodafone does not disclose its media budget, but is estimated to spend up to £600m a year and it is thought that up to a third is now being in-housed.
The company has said in-housing has driven efficiencies and it is under pressure to cut costs.
Nick Read, Vodafone's group chief executive, said this week that he will give up 20% of a planned, long-term share award in the face of investor disquiet because the stock price has slumped.
Vodafone operates in 25 markets, including Australia, Germany, India, South Africa, Spain and the UK.
WPP's MEC, which later became part of Wavemaker, beat Carat in the final round of the last review in 2014. Omnicom's OMD was the previous incumbent.