100 Trying: Caller is attempting to make the call180 Ringing: Call is progressing183 Progressing: Call is progressing. Ringing is normally heard.200 OK: Successful request. During a call invite this means the start of the call404 Not Found: Request URI isn't found. This could be a bad number or extension. 407 Proxy Auth Required: Invite received but was missing the authentication. This is expected. Remote party should respond back with authentication486 Busy Here: Called party is busy503 Service Unavailable: System is undergoing maintenance or is temporarily overloaded. Expected behavior is to move to next resource. Different endpoints and firmware will respond differently. This is sometimes configurable.606 Not Acceptable: Some aspects of the session description are not acceptable. This could be requested media, bandwidth, or addressing style.