For yr ref:
The CUG feature enables subscribers and PABXs, connected to a PLMN and possible also to other networks, to form closed user groups (CUGs) to and from which access is restricted.
The CUG feature refers to a set of facilities centered around a group of users who want to restrict their usage of the network to communications within the group.
A typical application of the CUG feature is for companies, providing mobile stations to their employees for professional use only. It is then possible to limit for each employee the outgoing
calls, or the incoming calls, or both, to calls within the group.