CUE is as the CME a small business voice mail solution that you can easily deploy on a router. Most of the time, CUE will reside on the same chassis than the CME but for some implementations, you can split both. The only backdraw is that you must have a LAN connectivity between your elements.
CUE is interesting because it doens’t provide only voice mail but can also provide a small Auto-Attendant (AA). The integration between CUE and CME is done via the SIP protocol, while the integration with a Call Manager will be done via JTAPI ( CTI-QBE). It can be the case for implementation where you need to provide Unity backup on SRST location).
CUE will be supported by Network Module (NM-CUE) or as an Advanced Integration Module (AIM-CUE).