If it is properly configured, then there is no need to use a star prefix. Log into the OBiTALK web portal. On the section for your OBi devicde, click SP2. On the main settings page, simply put a check mark in the box to "Use this service for emergency 911 calls" and click "Submit".
You should be able to dial 933 from the phone attached to the OBi to test that it's working.