Is it permissible to charge the company for the cost of using a personal phone for work calls at a rate of 50 piasters per minute, knowing that the cost per minute on the personal phone is 20 piasters, by analogy with the price per minute in a regular central exchange that generates profits?

It is not permissible to charge the company for minutes at the exchange's rate and take the price difference without their permission. The worker's ruling is that of an agent, and an agent is not permitted to buy from himself for his principal except with permission. As for the price of calls, either the company accepts the exchange's rate or you reach a settlement with them on a specific price. If they do not agree, then you have the right to the customary price.