Is using the company's phone for personal calls impermissible (haram), knowing that there are no bills for these calls?

If you know that the employer permits the use of the work phone for personal purposes, or if it is customary to do so, then there is no harm in you using it, and seeking permission is not necessary. However, if you know that they do not approve of this, or if custom dictates otherwise, then it is not permissible to use it for your personal needs except with their explicit permission.