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Yes, the ATLAS can be used to simulate a PRI to your PBX. The configuration will be very similar to what you currently have for the RBS T1 except under the DIAL PLAN in the USER TERM you will set the SIG to be PRI instead of RBS. Then under the IFCE CONFIG you will have options for the different switch types the ATLAS supports. The default is National ISDN (NI-2). You can also set the number of channels (the default is 23 channels plus the D-channel). From the ATLAS to your PBX should be a T1 cross-over cable. This should be the same pin-outs as you currently have running from the ATLAS to the NETWORK port of the TA6XX.
The TA6XX does not support PRI so you will not run to it first. You will run directly from the ATLAS to the PBX.
Under the DIAL PLAN, the configuration options are based on what the ATLAS is terminating to (NETWORK TERM, and USER TERM). So when the ATLAS interface is configured in the USER TERM it is actually acting in the NETWORK role because it is terminating to "user" equipment. When and interface is configured in the NETWORK TERM it is acting in the USER role because it is terminating to "network" equipment.
If you have more than one T1 interface available on the ATLAS you can keep your RBS T1 to the TA6XX and simply add a new PRI entry for the PRI to the PBX and then allow the ATLAS to route calls between them (this, of course would require you to configure the IN#ACCEPT for each entry so the ATLAS can route the calls to the appropriate interface).
Hope this helps,