Is it permissible to use a car and phone designated for work, with the knowledge of the managers, for personal purposes, and does the common saying, "Whoever is preoccupied with the affairs of Muslims to the exclusion of attending to their personal matters shall be provided a servant from the Muslim treasury," apply to this?

The phrase "who is preoccupied with the affairs of Muslims to the neglect of his own matters" is accurate, and it refers to those who manage the general affairs of Muslims, such as rulers, leaders, ministers, judges, and muftis. The Prophet, peace and blessings be upon him, permitted a person working in the affairs of Muslims to acquire a wife, a servant, and a dwelling if he did not possess them. As for working in a private company for a known wage, this does not fall under that category. One must fulfill the contract and use the company's phone and car according to the terms of the contract.