Firmware version: 188.8.131.52
Model: Standard V6, U-Series
Call dropping after 5 mins, then SIP provider response "420 Bad Extension".
Further analysis indicated that the "timer" field is unsupported.
Customized the script to disable the timer require.
Step 1: login device via ssh
Step 2: enter the following command
Step 3: Input letter "i" and the paste the following script
Step 4: Press "ESC" on keyboard and Input ( :wq ) and Enter
Step 5: Make any change on web and click the "Apply Changes" button or reboot the device to take effect the script.