Closed User Groups are groups of GSM mobile telephone subscribers who can only make calls and receive calls from members within the group. This service is not usable with the short messaging service (SMS) system.
A subscriber may:
- Be a member of more than one but not more than ten Closed User Groups.
- Be permitted to make calls outside of the Closed User Group (Outgoing Access).
- Be permitted to receive calls from outside of the Closed User Group (Incoming Access).
- Be allowed to make emergency calls irrespective of the group subscription.
If the user is a member of multiple Closed User Groups there will be a Preferred CUG assigned by the network that will be used by default. However, it is possible on a per-call basis to specify a different Closed User Group (of which the user is a member) for the call. It is also possible on a per-call basis to suppress the use of the Preferred CUG i.e. act as if the user is not a member of the Closed User Group, and to suppress the Outgoing Access permission i.e. to insist that the call only go through if the destination is a member of the CUG.
When an incoming call is received it is possible for the network to indicate the Closed User Group that is being applied to the call to the called user.
For example:
Mr Rao, a senior member at a Pizza delivery outlet, could be a member of two closed user groups:
Mr Rao's Preferred CUG would be that of his team. But based on whom Mr. Rao is calling, he can either suppress or enable the preferred CUG. Also, when Mr Rao receives a call, the network would indicate which user group the call originated from.
As can be seen, this Supplementary Service is restricted in use only by organizations, and is not for use by the general public. However, there are handsets that support Closed User Group applications.