Editing Class of service (section)

==Voice services==
As related to legacy telephone systems, COS is often used to define the permissions an extension will have on a [[Private branch exchange|PBX]] or [[Centrex]]. Certain groups of users may have a need for extended [[voicemail]] message retention while another group may need the ability to forward calls to a cell phone, and still others have no need to make calls outside the office. Permissions for a group of extensions can be changed by modifying a COS variable applied to the entire group.

COS is also used on trunks to define if they are [[full-duplex]], incoming only, or outgoing only.{{cn|date=November 2018}}