PSTN信令通过使用手机产生的声调来开始或结束通话。
PSTN signaling USES tones generated by the phone to begin or end a call.
SIP是一种应用层控制信令协议，用于创建、修改和结束与一个或多个参与者的会话。
The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying and terminating sessions with one or more participants.
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