I have seen many Telco lines where "Positive Disconnect" is an option on trunks and you need to request it from the carrier, it is typically called CO Disconnect, Positive Disconnect, Calling Party Disconnect, or Release Confirmation. What should happen on a business analog line is there should be a break in the voltage from the telco for anywhere 200-1000 msec shortly after the far end disconnects, or the telco sends a CPC Tone (Calling Party Disconnect Tone) to confirm the remote end disconnected and the 3300 will see this and act appropriately. Note that some ATA devices (like what cable companies or SIP providers use to give you analog dialtone) do not support positive disconnect. I have also seen some carrier that charge an additional fee for this "feature".
Like Ralph posted above, the disconnect timer in the CO Descriptor should be just lower than the telco disconnect timer (if Open Loop Disconnect is used), but if you shorten it too much lines can disconnect in the middle of conversation, for example if there is an aerial cable with a bad splice and the wind blows and opens the connection momentarily. Also, I always disable the "Ignore far end disconnect" AND "Release Supervision Expected", and typically enable "Calling Party Disconnect" but in some instances it needs to be disabled for the 3300 to release properly, I know that's counter intuitive but it happens sometimes.
With lines from the local cable company, I need to lower the Disconnect Timer in the 3300 to 350ms for the lines to consistently release properly, at the default of 500 they occasionally get hung up in the system.
