Northern Telecom Inc is celebrating a $280m, 40-month contract to supply United Telecommunications Inc with digital switching systems for routing local and long-distance calls: the agreement, which is retroactive to September 1 last and runs to December 31, 1990, is for the DMS-10 rurual and DMS-100 urban local telephone exchanges; the DMS-100 offers Meridian Digital Centrex office services support, and both exchanges will be able to be retrofitted with support for ISDN.