When you log into the OBi directly (at the IP address given when you dial ***1) and go to System Management -> Auto Provisioning, you should see a section named ITSP Provisioning and another named OBiTalk Provisioning. In both of these sections, set the Method to Disabled, click Submit. Reboot the device, then confirm that X_SkipCallScreening is checked.
If you still have trouble, report whether you are prompted to press 1, or if you hear silence, but pressing 1 connects the call anyway.