Call hangup/disconnect problem with 2 MultiVOIP 800's connected FXO - FXO, both sides to PBX Extensions.

In order for PBX extensions to hangup properly with FXO extensions the PBX needs to support the current loss feature. If the PBX does not support current loss then either tone detection or silence detection can be used. If your PBX has E&M cards this may be a better alternative for cleaner disconnects...otherwise you may want to look into your PBX setup to see if a disconnect on current loss option can be enabled.