Correct Answer: INVITE, followed by one or more provisional responses (such as 100 Trying and 180 Ringing), then a 200 OK final response, followed by an ACK from the calling party
Explanation: Normal SIP call setup follows INVITE, provisional responses (100 Trying, 180 Ringing), a 200 OK, and an ACK, unlike an INVITE immediately followed by BYE, REGISTER immediately failing with 404, OPTIONS immediately canceled, SUBSCRIBE/NOTIFY with no INVITE, or PUBLISH immediately erroring, none of which describe call establishment.
Correct Answer: INVITE, followed by one or more provisional responses (such as 100 Trying and 180 Ringing), then a 200 OK final response, followed by an ACK from the calling party
Explanation: Normal SIP call setup follows INVITE, provisional responses (100 Trying, 180 Ringing), a 200 OK, and an ACK, unlike an INVITE immediately followed by BYE, REGISTER immediately failing with 404, OPTIONS immediately canceled, SUBSCRIBE/NOTIFY with no INVITE, or PUBLISH immediately erroring, none of which describe call establishment.