Possibly, as you are hanging up you are "bouncing" the receiver on the hook switch, which may look like a "flash" to the OBi. You can confirm this by, at the end of a call, pressing the hook switch with your finger, holding it for two seconds, before replacing the receiver.
If that prevents the trouble, try tightening up the criteria for a hook flash. For example, for the Phone port, try changing HookFlashTimeMax from 900 to 750, and HookFlashTimeMin from 70 to 250.
