For example, when caller calls callee through third party app, both phones are ringing. If clicking the "Answer" button, I can send "200 OK" to caller to stop rining. The caller will send "ACK sip:xxxxx" back right away, since the callee actually don't pick up the phone, the call goes missed call. If I block this "ACK sip:xxxxx" to callee, it'll still ring. After picking up, they can talk. The problem is that what sip command I can send to stop callee's phone ringing? After picking up, it can still send out "200 OK" and set up RTP connection. In traditional phone, we can control this by PBX switch API. Does IP PBX have this API? Can 3CX IP PBX do this and how?