Is money acquired from a phone competition, entered for entertainment, considered unlawful if a monetary prize is won? What should be done with the remaining money, and how can one atone for this sin?

If the call cost is normal and there is no increase due to the competition, there is no harm in benefiting from the prize if the the questions are useful. However, if the cost is higher than usual, it is considered prohibited gambling, and the prize must be disposed of by spending it on Muslim welfare or on the poor and needy. A poor person in need is permitted to benefit from it to the extent of their need.