Does forgiving the employer for using the company phone for personal matters absolve one of sin, or must the amount be repaid? And is there a worldly or otherworldly punishment for it?

Using a work phone for personal purposes without permission is impermissible and is considered wrongfully consuming people's wealth. It requires repentance and paying the cost of the calls or seeking permission. As long as you are remorseful and repentant, and the company owner has forgiven you, you are not required to pay the amount, because the owner of the wealth has waived it. Your sincere repentance erases the sin, for Allah is Forgiving and Merciful to those who repent, as He, the Almighty, says: "And indeed, I am Forgiving to him who repents and believes and does righteousness and then is guided" (Taha: 82).