The appointment reunites Vodafone group director of corporate affairs Simon Lewis with Finsbury, the agency he retained as comms chief at Centrica.
Lewis said Finsbury would work in partnership with Tavistock but declined to rule out taking the latter's brief in-house, saying this was 'one of the options' under consideration.
Sources close to last month's review said Tavistock's 14-year relationship with the mobile phone company would be gradually wound down.
Although Tavistock was among five firms that pitched for the business at the beginning of February, it is understood that the final pitch came down to a two-way battle between Finsbury and Financial Dynamics.
Tavistock chairman Jeremy Carey declined to comment.
It is thought that Finsbury will bill around £300,000 for the account, which is being led by Finsbury partner Rollo Head.
Working with Finsbury co-founder Roland Rudd, Head leads a team that will advise Lewis and chief executive Arun Sarin on the company's international corporate and financial comms strategy.
Finsbury will also handle media and analyst relations, which took a knock last month after the company revealed its Japanese operation, which accounts for 20 per cent of group global revenue, had lost 58,700 subscribers in January.
Analysts have also pressed the company about falling average revenue per user.
Vodafone's pre-tax profits for the six months to 30 September 2004 were flat at nearly £5.4bn, up just £28m from the same period in 2003. Vodafone is to present its full-year results later this month.