The agency will be responsible for all B2B PR at national, regional and specialist level.
The appointment is a first agency hire for Kingston inbusiness and comes 18 months after the company was formed following a series of subsidiary mergers.
Mantra co-founder and joint MD Lawrence Dore will head a 'sizeable' team on the account and report direct to the client's MD Jane Hannah.
Kingston inbusiness is the B2B division of Kingston Communications, and was formed in April 2001, comprising the previously separate operations of Torch Telecom, Kingston Business Communications, Kingston Voiceware, Kingston Internet and Kingston Mobile.
Kingston Communications is the third biggest telecom company in the UK and recently announced a 40 per cent year on year turnover rise to £316m.
Kingston inbusiness contributed £197m to that total - a yearly increase of 55 per cent. The B2B division provides telecoms products exclusively for businesses, offering products and services including voice, mobile, data and internet services, equipment installation and maintenance.
The Hull-based Kingston Communications is famous for its white telephone boxes, a remnant from its past as a separate telecoms provider outside the old British Telecom monopoly.