Is it permissible to sell phone cards allocated by the company to an employee for their personal use, in order to benefit from their value for private matters, especially if they are not used for calls?

If the company policy allows the employee a communication allowance, then there is no harm in utilizing surplus call credit and benefiting from its value. However, if the allowance is specifically designated for company calls, it is not permissible to take the surplus unless with the consent of those in charge of the company.