Does using company property for personal gain, such as company cars or phones, or making personal calls from company phones, constitute open sin that necessitates repaying monetary amounts to the company?

It is not permissible for an employee to use company property except with the permission of the employer or according to custom. The employee must return what he took from his workplace, whether in cash or in kind, if it is still available, or its value if it is no longer available. What is established by custom can be deviated from if the contracting parties explicitly state so in a manner consistent with the intent of the contract.