protocol. The SMSC is a central routing hubs for Short Messages. Many mobile service operators use their SMSCs as gateways to external systems, including the Internet, incoming SMS news feeds, and other mobile operators (often using the de facto SMPP standard for SMS exchange). The SMS standard is also used outside of the GSM system; see the main article for details. Supplementary Services See also GSM codes for supplementary services. Call forwarding. Barring of Outgoing Calls. Barring of Incoming Calls. Advice of Charge (AoC). This GSM service estimates the call cost for display on the user's mobile phone. This helps users by preventing bill shock and reduces the load on the mobile network operator's customer service department. However, in practice, this GSM service is rarely used because the calculation is more complex than the standard allows. Instead, some operators provide subscribers with balance and/or call-cost notifications sent at predetermined times, such as at the beginning or end of every call. These notifications may be read out loud using a speech-synthesis system, or may be transmitted in textual form. Call Hold. Call Waiting. Multiparty service. Calling Line Identification Presentation (CLIP)/ Restriction (CLIR). Closed User Group (CUG). Explicit Call Transfer (ECT). This service allows a user who has two calls to connect these two calls together and release its connections to both other parties. See also GSM USSD codes - Unstructured Supplementary Service Data: list of standard GSM codes for network and SIM related functions References External links GSM Call Flows and Sequence Diagrams Detailed call flow diagrams describing GSM call setup, location update and handover. GSM Call Flows and Tutorial Detailed call flow diagrams GSM standard