Is using the company's phone lines for personal calls – the cost of which is covered by the company – forbidden (haram) or permissible (halal), given that the user does not receive their full rights from the company? And how can one atone for this action if there is any doubt about it?

The worker is entrusted with what is under his hand, and it is not permissible for him to use it except for what he has been authorized for, because safeguarding the trust is obligatory.

As for you not receiving what your colleagues receive, there are two cases: 1. If what you receive is specified by a contract between you and them, then you are not allowed to demand more than what is in the contract. 2. If the contract between you and them was for a specific amount and they breached it, then you may take its equivalent without increase. This is known as "seizing one's right," provided that it pertains to past months without their informing you. However, if they informed you and you accepted that, then it is like the first case.