In the present scenario when Telephony systems are moving to a common wiring plant the end station of a LAN or IP- based PBX must be simple to use, familiar and relatively cheap. While the H.323 recommendations are pretty expensive, an H.323 proxy can be used to communicate with the Skinny Client using the SCCP.[para] Here the telephone is a skinny client over IP, in the context of H.323. A proxy is used for the H.225 and H.245 signaling. Skinny messages are carried above TCP and use the port 2000 at the same time the skinny client (i.e. an Ethernet Phone) uses TCP/IP to transmit and receive calls and RTP/UDP/IP to/from a Skinny Client or H.323 terminal for audio.

SCCP (Skinny Call Control Protocol) is a proprietary terminal control protocol originally developed by Selsius Corporation. It is now owned and defined by Cisco Systems, Inc. as a messaging set between a skinny client and the Cisco CallManager. Examples of skinny clients include the Cisco 7900 series of IP phone such as the Cisco 7960, Cisco 7940 and the 802.11b wireless Cisco 7920, along with Cisco Unity voicemail server. Skinny is a lightweight protocol which allows for efficient communication with Cisco CallManager.[1] CallManager acts as a signaling proxy for call events initiated over other common protocols such as H.323, SIP, ISDN and/or MGCP.

A skinny client uses TCP/IP to and from one or more Call Managers in a cluster. RTP/UDP/IP is used to and from a similar skinny client or H.323 terminal for the bearer traffic (real-time audio stream). SCCP is a stimulus-based protocol and is designed as a communications protocol for hardware endpoints and other embedded systems, with significant CPU and memory constraints.

Cisco acquired SCCP technology when it acquired Selsius Corporation in 1998.[2] As a remnant of the Selsius origin of the current Cisco IP phones, the default device name format for registered Cisco phones with CallManager is SEP -- as in Selsius Ethernet Phone -- followed by the MAC address.

Other companies like Symbol Technologies and SocketIP have implemented this protocol in VoIP Terminals (phones) and Media Gateway Controllers or Softswitches. Open Source implementations of SCCP/Skinny exist and are now part of the Asterisk (PBX) system.

A company named IPBlue has created a software phone (soft phone) which uses SCCP for signaling, too. This phone in fact appears to the Cisco CallManager server as a 7960 hardware phone.

In addition, Cisco has come out with its own version of a skinny softphone called Cisco IP Communicator as well as SIP-based softphone called Cisco Unified Personal Communicator. Previously, Cisco had a JTAPI/CTI version of a softphone called Cisco IP Softphone.