Issue Description：
According to message tracing, messages for inviting external users cannot be routed to terminals, or the USM returns exceptions to messages sent by terminals for joining conferences.
Possbile reason：
The conference routing function is faulty.
Solution :
Trace IMPU messages to determine the abnormal NE.
Trace IMPU messages based on the user number to determine the NE that sends the first exception message (such as the BYE message or an error code that is greater than or equal to 300). If the USM is abnormal, go to the next step. If another NE is abnormal, locate the fault by referring the help document of the NE.
- Check the error code manual for the call failure cause.
Analyze the error code that is greater than or equal to 300 and the warning information by referring to the USM error code manual. Obtain the cause and handling suggestion.
