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Presidential candidates have to be optimistic - right down to their telephone numbers. That explains why the number of Walter Mondale's campaign office is 625 -1600 - referring to that big residence at 1600 Pennsylvania Avenue. Not to be outdone, Sen. John Glenn's headquarters is reached at 783-1984, and Sen. Ernest Hollings changed his number to 393-1984. That's the year they hope to win the privilege to be reached at 456-1414 - President Reagan's current number.