The following description of the H.323 call flow is correct

People convert the corresponding mechanical vibrations of sound into electrical signals, which are collectively referred to as audio signals.

The role of RTP in VoIP mainly includes: real-time transmission of media information, elimination of jitter, card performance, prevention of packet loss, in a certain situation can be transmitted DTMF signals, tones and signaling.

In the SIP protocol, when the response message is "1xx", it means that the request message has been received and is being processed.

Which of the following signaling signalings can the gatekeeper issue address resolution requests to other gatekeepers?