3PCC app control IP hard phone's connection

[James123]

For example, when caller calls callee through third party app which is registered as SIP proxy, both phones are ringing. If clicking the "Answer" button on third party app, 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 to missed call.
If I block this "ACK sip:xxxxx" to callee, it'll still ring. After picking up, they can talk. But users cannot accept that phone still ring after clicking "Answer" button.

What SIP command can I 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. Which IP PBX provides this API?

Thanks in advance.