You are troubleshooting a SIP routing issue and you have access to the SIP call flow processed by the Session Border Controller. Which two steps are…

A: It is necessary to locate the SIP session to troubleshoot and check if there is a 404 Routes Not Found SIP error message. B: It is recommended to look for 183 Session Progress SIP error messages that indicate routing issues. C: It is recommended to check the called number by looking at the Request-URI of the INVITE message and making sure it is a valid number to be routed. D: It is recommended to find the CANCEL method received by the Session Border Controller which typically identifies a SIP routing issue. E: It is necessary to find an initial INVITE received by the Session Border Controller which gets a 503 Service Unavailable SIP error message.