Is the excess mobile phone allowance given to an employee permissible (halal) or forbidden (haram), and is its use for purposes other than official work calls permissible?

As long as your manager at work has authorized you to benefit from the card, and even requested the company to pay it to you, then there is no harm for you in benefiting from it, provided that he is authorized to do so by the authority that appointed him, and that he has the right to grant permission for it. The manner of benefiting from it depends on what the manager specifies; if he permits you to use it for your personal affairs, then there is no harm. However, if he restricts its use to work matters, then you are not permitted to use it for anything else. But if the manager is not authorized to do so, then his permission is not valid.