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There shouldn't be any configuration in the ATLAS that will effect your application. What do you hear on the butt set when dialing out on one of the FXS ports? The ATLAS should send out a SETUP message and the Telco should send back a CALL PROCEEDING, to acknowledge the SETUP and confirm it is processing the call, followed by an INBAND AUDIO AVAILABLE which cuts the talk-path through so the "tone prompt" can be played over the talk-path. You would have to monitor the ISDN messages to confirm this. (In the ATLAS under EVENT LOGGING you will have to set the ISDN L2 FORMATTED events to INFO.)
If the ATLAS receives an INBAND AUDIO AVAILABLE message from the Telco, then talk-path is cut through so the caller can hear the audio message being played. If it is working PRI to PRI then it should work PRI to FXS - the principle is the same.
The ATLAS will either play ringback on the FXS port or cut talk-path through to play audio from the carrier (depending on the ISDN message received from the Telco - either ALERTING or INBAND AUDIO).
If you can capture the Event Logs in the ATLAS, that may help.
Thanks for the prompt and detailed response Patrick.
>What do you hear on the butt set when dialing out on one of the FXS ports?
I'll setup logging when our traffic is much lower. Possibly this weekend.