Issue: For some International calls, "SPEECH" must be configured as the setting in the ISDN Transfer Capability parameter or you may experience random International and Toll Free calls failing.
Resolution: This parameter is the "ISDN Transfer Capabilities" that can be found on Trunk Settings Page. By default, this parameter is set to Audio 3.1. You can change this to Speech. You can get to this paramted by navigation to:
Under VoIP, PSTN, Trunk Settings, ISDN Transfer Capabilities.
This should be a standard configuration setting for deployments.