BT will launch the service - available on the number 118500 - at midnight on 10 December, when the market opens to competition.
Simon Lubin, head of marketing for BT Directories, part of the BT Retail arm, confirmed the appointments and said a 'whole range of PR activity' is planned to 'get 118500 to the forefront of people's minds'.
Cake has been appointed to handle consumer PR and FH will give advice on corporate and strategic PR.
The two agencies worked in tandem on the launch of BT Broadband in September.
Lubin, who described the 118500 service as the 'new-improved 192', stressed that 192 is 'alive and well' and will continue until August 2003.
Director Simon Matthews heads an account team at FH, while at Cake, account director Thea Gray, reporting to MD Darren Thomas, heads the work.
Cake is currently working on a consumer press relations blitz for BT Broadband's sponsorship of the second series of Celebrity Big Brother, which kicked off this Wednesday, in conjunction with Channel 4's in-house PR team.
FH was appointed as the core agency for BT Retail, the telecoms giant's largest division, in August (PRWeek, 2 August). It won a four-way pitch to secure that business.
A number of firms are poised to enter the £700m directory enquiries market. The Number, the UK brand of US firm InfoNxx, has hired Jackie Cooper PR and Le Fevre Communications to handle its PR (PRWeek, 8 November).
In addition, 118UK - the UK brand of Dublin-based directory assistance firm Conduit - is also set to launch its own service.