1. Capture logs and check the signaling, from the signaling we can see U1980 sent out the first called number but didn’t get any response from PSTN.
2. Check with PSTN, and found they are use DTMF type signaling, but default U1980 was using MFC type.
3. Modified U1980 signaling type to DTMF, the command was: config modify board slot x type dtu trunkport x officeno x sendnumbertype dtmf
4. Test again, and the issue was resolved.