Step 1 Check the SIP trunk.
Run the show peercompant command. It is found that the commstate value is OK. The SIP trunk works properly.
Step 2 Capture SIP data packets. It is found that the peer end returns the 401 message, as shown in the following figure.
Step 3 Use the UCMaint to capture data packets on the U1980. The following figure shows the information found.
Step 4 Based on SIP log analysis, the peer end returns the 401 message for the 200 OK message to release the call. Therefore, you must confirm with the peer end.
Step 5 Confirm with the peer end. It is found that the peer end uses open source PBX software, which poses high security requirements. The peer end forbids calls through anonymous SIP trunks.
Step 6 Ask the peer end to allow calls through anonymous SIP trunks. The fault is rectified.