What is the ruling on using a work phone for non-essential personal purposes, and what is the evidence for its prohibition from the Qur'an and Sunnah for someone who does not know the employer to seek their permission?

It is permissible to use a work phone for personal purposes with the permission of the employer or if it is customary. If the employer does not permit this, the violator must repent and estimate the cost of the private calls and return it to the company by any available means, or purchase work supplies for the company equivalent to the value of those calls.