No ring-back is heard

I have a SIP trunk from FreePBX to an Adtran TA 904. When calls are sent to the Adtran, there is no ring-back being heard by the caller.

Has anyone run into a similar situation? Is there a good place to read up regarding problems like this?

FreePBX - Inbound Route -> Ring Group -> Extension

Adtran TA 904: A.2.05 - connected to FXS ports

Thank you,