In the "settings" menu of IVM under the "devices tab", did you set the "end of tone settings" to harware detection, software detection or both?
Here's info from the NCH Site
Some voice modems incorrectly detect the end-of-call tone. On the IVM, use the "End-Of-Call Tone Detection Settings" (on the Settings - Devices tab) to correct defects. (a) If the modem does not detect end-of-call when it should, tick the Enable Software Based Detection box. ( If the modem detects end-of-call when it should not (and therefore hangs up), untick the enable detection during playback option.
Here's how to set your board...
Configuring your Intel / Dialogic Card to Detect Disconnect Tones:
Some PBX's send a tone to tell the telephony device that the remote call has disconnected, for example the DTMF "D". In order to take advantage of this you will need to configure the Dialogic drivers to recognize the disconnect tone.
First you will need to tell the device to listen for the tone. Then you will need to run the PBXpert configuration utility to allow the device to learn which tone to listen for.
Run the "Dialogic Configuration Manager".
If the service is running then stop it.
Select your telephony device.
Configure the device (double click on it).
Select the "Misc." tab.
Modify the "DisconnectTone" option to have a value of "Yes".
Run the PBXpert utility
Try this out - let know - good luck!