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Calling Party Control of Disconnect
Calling Party Control (CPC) is how the calling end notifies the called end that the established connection is no longer needed. The CSP provides this capability with the CPC Detection message. The CSP scans for dialtone from the distant end to indicate that it has been released.
CPC is used only on trunk interfaces that provide no release information through signaling. For example, the CSP commonly uses CPC Detection on T1 FXO-LS or analog FXO-LS lines that provide no loop current break to signal a release.