There are three ways of separating calls:
By using an “escape code”: the user presses a specific key to determine the call type
By defining lists of authorized numbers, even abroad
By defining time slots (e.g. in national use, you configure a mobile number so that all calls between 9:00 and 17:00 are paid for by the company)
For the sake of consistency, you can implement the same rules for fixed outgoing calls by programming your phone exchange, and for mobile outgoing calls by implementing Private Use. Example: calls to a specific partner will always be paid by the company.