Networking: RBT server --> SIP trunk --> SoftCo --> ISUP trunk --> Carrier's network
Version: SoftCo V100R002C04SPCa00
Symptom: Internal users can hear normal RBT when making internal calls. However, when external users call SoftCo users, the external user can talk with the called party but cannot hear the normal RBT or ringback tone.
Step 1 Capture SoftCo logs.
The SoftCo is normal.
Step 2 Capture and analyze network packets on the RBT server.
It is found that the media IP address carried in the SDP field of the 183 message returned by the RBT server is different from the actual IP address that plays announcements. The media IP address is 220.127.116.11, and the source RTP IP address to play the RBT is 18.104.22.168.
Step 3 Change RBT server configurations and write the source IP address into the DSP stream.
ISUP trunk configurations are incorrect. When the ISUP trunk is used, the local end (called party side) plays ringback tone for the calling party by default.
Internal users can hear normal RBT because the RBT server directly sends the media stream to the IAD. However, when external users call internal users, the RBT server must forward the media stream to the SoftCo at first. The SoftCo discards the media streams because the media IP address is different from the actual IP address that plays announcements. Therefore, external users cannot hear normal RBT.