What is the ruling on working and profiting from an Islamic phone application for a fee, and is a person rewarded for that when Muslims use it?

There is no objection to profiting from useful applications. The reward for Muslims using them depends on the worker's intention and his aim to assist in obedience and guide towards good, for the Prophet, peace and blessings be upon him, said: "Deeds are but by intentions, and every man shall have but what he intended." The desire for profit does not contradict the hope for reward; rather, the reward for one who does it purely for the sake of Allah is more complete and greater. Ibn Taymiyyah said: "The books that are loved and for which a person is rewarded for writing, whether he writes them for himself or writes them to sell them... in both cases, he is rewarded."