1. Capture call logs of U1980
2. Check the log, and found that the call was established after U1980 send A6 message.
3. Modify the R2 trunk configuration to enable group B signaling., the command is: "config modify cassignal casname xxx needbgroupsignal yes"
4. Then test incoming call again, the Mobile phone can hear the ringing back tone, the issue is resolved.