Yealink SIP-T4X Administrator's Manual page 427

SIP uses six request methods:
INVITE—Indicates a user is being invited to participate in a call session.
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ACK—Confirms that the client has received a final response to an INVITE request.
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BYE—Terminates a call and can be sent by either the caller or the callee.
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CANCEL—Cancels any pending searches but does not terminate a call that has

already been accepted.
OPTIONS—Queries the capabilities of servers.
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REGISTER—Registers the address listed in the To header field with a SIP server.
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The following types of responses are used by SIP and generated by the IP phone or the
SIP server:
SIP 1xx—Informational Responses
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SIP 2xx—Successful Responses
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SIP 3xx—Redirection Responses
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SIP 4xx—Client Failure Responses
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SIP 5xx—Server Failure Responses
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SIP 6xx—Global Failure Responses
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Appendix
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