Catalysis has been briefed to handle UK trade and business-to-business PR, focusing on media relations within retail, finance, utilities, insurance and public sector publications.
The brief is thought to be worth about £6,000 a month.
The decision to bring in Catalysis had been delayed by the firm's potential £1bn acquisition by Vodafone, which at the time of PRWeek going to press was still ongoing.
Head of PR at Cable & Wireless Worldwide Will Cameron said: 'The deal with Vodafone came out of the blue and put our decision to appoint after the pitch on the backburner.
'But now we have appointed, it's about going back to basics and re-engaging with our core IT and industry press contacts so that we are at the top of everyone's call lists.'
Cameron said the brief was 'very much about connecting with trade media in a traditional way', with the focus on social media taking place further down the line.