Which standard protocol is utilized for signaling between the Cisco Unified Communications Manager Express router and the Cisco Unity Express module?

SCCP (Skinny Client Control Protocol) is a Cisco-proprietary protocol used between CME and IP phones, not for integration between CME and the CUE module.

HTEMS is not a recognized or standard VoIP signaling protocol and has no role in any Cisco UC integration.

SIP (Session Initiation Protocol) is the industry-standard VoIP signaling protocol used for the CME-to-CUE integration, enabling CME to forward calls to CUE and exchange Message Waiting Indicator (MWI) notifications via SIP NOTIFY messages. This open standard approach allows interoperability and is explicitly required in CME-CUE dial-peer configuration.

XCP (Extensible Communications Platform) is associated with Cisco Unified Presence and instant messaging services, not with CME-to-CUE call signaling.