Hi Antonio,
If the UC320W does not see the CPC disconnect tone from the teleco, it doesn't hang up the call until the maximum voicemail message length of 10 minutes is reached.
Make sure that you have run the FXO Impedance Matching tool on each of the FXO lines. You can find more info in the UC320 Troubleshooting Guide .
Next, if you are still having problems, contact the telco and make sure they are sending a CPC disconnect tone length of at least 200 milliseconds.
If you are still having problems, you will need to contact the Cisco Small Business Support Center where they can capture the audio and verify the telco is sending the correct tone and duration.
Hope this helps.
Chris