The telecoms service provider, which supplies companies including Cable & Wireless, is seeking to move on from its collapsed sell-off in August.
Bell Pottinger Corporate & Financial will continue to advise the Coventry-based company.
But the need for a ‘stronger focus on the trade and telecoms industry' has now prompted the appointment of GBCS, a telecoms specialist, according to Telent communications and marketing services director Helen Johnston.
The agency won the account after a competitive pitch against undisclosed agencies.
GBCS will target media in countries such as Ireland and Germany, plus the UK.
The bulk of the former Marconi was sold to Ericsson for £1.2bn in 2005.