All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The question that you are asking includes two matters:
1) If the medical service mentioned is included in the contract signed by the company and the employee, then that is not permissible unless it is a limited deduction from the wages of the employee whether or not he needs the service. But if there is no specified deducted amount and the employee could get medical treatment with a very cheap or a very expensive amount, or not be treated at all, then the deal in his contract is unclear and there is deception in it; and this matter is forbidden in Islamic Law. Imaam Muslim reported that the Prophet forbade contracts that entail deception.
2) If the medical service mentioned is a gift from the company, in which case accepting such a service is dislikeable because the expenses are covered by the money previously paid by the company and the money of the insurance company. So it is mixed money from lawful and forbidden income, and accepting the donation of money that is mixed with lawful and forbidden income is dislikeable according to the opinion of the majority of scholars. This applies of course to the employee who benefits from such a treatment and has no relation with the insurance contract. However, if he is given a choice whether or not to join this contract and he joined it, then it is not permissible for him to benefit from the medical treatment except for the amount that he had previously paid or which the company paid on his behalf.
Finally, the issue of insurance has already been discussed in Fatwa: 81425, so please refer to it for more benefit.
Allaah knows best.