What is the acceptable limit for using a personal phone at work after obtaining the manager's permission, and is it necessary to ask permission for every long phone call?

The permissible extent of using a work phone for personal purposes can be determined by a clear stipulation from the authorized person in charge, or by established custom. If neither of these exists, then usage must be limited to the absolute minimum possible, because the default rule for using a work phone [for personal matters] is prohibition.