How competitive is the long-distance telephone business? Sprint Corp. was forced to yank TV commercials for a new collect-calling service last week after MCI Communications launched a countermove. The Sprint ads, which aired only three times, said Sprint's 1-800-ONE-DIME service was cheaper than MCI's 1-800-COLLECT. That was true--until MCI lowered its rates to match Sprint's. A victory for consumers--but not for Sprint, which is scurrying along with ad agency J. Walter Thompson-San Francisco to prepare new commercials. Said MCI representative Brad Burns: "We weren't about to let a new product erode our market share."