Is it permissible for the questioner to keep a gift (phone) that was given to him when purchasing a PABX system for his work, knowing that he secured an additional discount beyond what was agreed upon, which benefited the employer?

The questioner is a private employee and an agent for the employer in purchasing the central unit. He is obligated to do what is best for his client, as he is entrusted. Therefore, it is not permissible for him to take the gift for himself unless permitted by the employer, because it was given to him due to his position, not his person. This falls under the category of gifts given to workers, regarding which the Prophet, peace and blessings be upon him, said: "Gifts of workers are ghulūl (ill-gotten gain)." He must inform the employer; if the employer permits him, then he may take it, otherwise, he must give it to the employer.