Is it permissible for an employee to use the company's work phone for personal matters, just as senior employees do without objection, knowing that each department pays the phone bill?

Whatever was designated for work remains so, and it is not permissible to use it for personal matters except with the permission of the employers, due to the Prophet's (peace be upon him) saying: "The wealth of a Muslim person is not lawful except with his good consent." The non-belligerent disbeliever in this regard is like the Muslim. Therefore, it is necessary to ask those in charge of this matter. If they grant permission, then that is excellent. Otherwise, it is not permissible to use any of it. The silence of everyone is not considered, unless they are the employers themselves.