American Telephone & Telegraph's plan to raise weekend and nighttime long-distance telephone rates by 12.5% would have become effective Sunday. The Federal Communications Commission ruled that AT&T would make too much profit on its measured toll service if it got the $150-million increase. Calls made during those periods cost 60% less than the same calls made during daytime hours. AT&T wanted to reduce the discount to 55%. MCI Communications, another long-distance company, had intended to raise its rates, too, but said it would hold off if the FCC blocked AT&T's increase.