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Session Initiation Protocol (SIP) servers can experience prolonged overload due to high rates of incoming service requests or partial network failures. SIP servers need to be equipped with some form of adaptive overload detection and control, in order to protect against high and unpredictable levels of demand and to keep response times low enough during processing overload to preclude customers abandoning their service requests prematurely. Many SIP servers have internal (local) overload control mechanism, where the overload servers reject enough new INVITE requests to maximize successful completion of admitted sessions. However such mechanisms can only protect the SIP servers against overload to a limited extent, and it cannot prevent congestion collapse. In this paper, we introduce Signal-Based Overload Control, SBOC, which does not require changes in the SIP protocol, as it is implemented at the sending (upstream) servers, consequently it does not impose processing burden on overloaded servers. Extensive simulations show that SBOC maximizes the goodput of the SIP network, and adapts appropriately under realistic overload profile.