Thank you for using the ADTRAN Support Forums.
Unfortunately, it sounds like you need the ATLAS to convert audio on a BRI to SIP/VoIP in order to send it over the Frame Relay link to the Cisco router. The ATLAS does not support VoIP. The ATLAS can do BRI to PRI switching, but not BRI to SIP.
Sorry, but I don't believe the ATLAS supports what you need.
Thank you,
Patrick