Dallas--The business-to-business unit of Sprint has ended its six-year relationship with Grey Advertising, New York. Sprint Business said the decision would allow it to expand Internet marketing initiatives. Advertising previously earmarked for Grey will go to one of Sprint's other agencies: Hal Riney & Partners, San Francisco; McCann-Erickson Worldwide, New York; D'Arcy Masius Benton & Bowles, St. Louis; US Interactive, Cambridge, Mass.; or VML, Kansas City, Mo. Sprint Business' annual b-to-b advertising budget is estimated at $50 million. The move underscores changes in telecommunications marketing: As companies move from the traditional long-distance carrier model, where mass advertising was best, Internet marketing that allows for explanation of complex products seems to be gaining favor. Grey will continue to handle Sprint's corporate brand advertising and media planning.