StartChannel(0)
re-invite
Dtmf End
180 Ring
FlashHook
call UserA（FXS1）
Set CNG/VAD/Codec/Ptime/PT/LSDP/RSDP/DTMF
200 Ok
UserC(FXS3)
Play CallHold Tone
invite
OpenChannel(1)
Bye
UserA(FXS1)
close RTP(1)
Set EC/FAX/Jb/gain
Talking
说明：本流程图体现呼叫等待、呼叫保持功能的实现方案，UserA/B/C三个用户可以在同一单板，也可以在不同单板，SIP信令控制流程未完整体现
StartChannel(1)
close RTP(0)
Stop Tone
Play RingBackTone
StopTone
Dtmf Start
Release
close channel(1)
Play BusyTone
UserB(FXS2)
阶段3 FXS1挂机
RTP control(inactive)
onhook
Send CallerId and Ring
Play SpecialDailTone
Talk
OpenChannel(0)
offhook
Play DailTone
Play CallWait Tone
Close RTP(0)
Send CallId
Enable DtmfDetect
阶段1 FXS1呼叫FXS2
collect dtmf
阶段2 SIP呼叫FXS1
offhook and Stop Ring
Dtmf 2
Set EC/fax/Jb/gain
close channel(0)
