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The ATLAS uses the IN#ACCEPT and OUT#ACCEPT in the DIAL PLAN in order to route the calls. So if you have 6-8 FXS ports that all have an IN#ACCEPT of "1200" then when there is a call to "1200" that comes in the PRI, the ATLAS will route it to the first available FXS port that will accept "1200". You can do this by having each FXS port listed separately or you can have one DIAL PLAN entry, with the Slot and "starting port" and under the IFCE CONFIG set the NUMBER OF PORTS to be 6 or 8. This way you are configuring all the ports identically in one DIAL PLAN entry. So all the ports will accept the same IN#ACCEPT, etc.
The IN#ACCEPT list can have more than one entry. So each FXS port (or hunt-group) can accept more than one number. If it is one port, than if that port is busy, the second call will get a BUSY, and if multiple ports are configured in a hunt-group, then the first available port will ring when any of the numbers is received.
For your last question, you can definitely configure one FXS port to have one specific number in the IN#ACCEPT list. As long as this is the only port with that specific number, then that is the only port calls to that number will be routed to. The ATLAS routes calls based on the "most specific match," so if you have "5555" as the IN#ACCEPT on FXS port1 and "555X" on FXS port2, a call to "5555" can only be routed to port1 since that is the "most specific match," but a call to any other 555X number would be routed to port2.
Hope this helps,
Since there has been no further discussion on this, I will mark it as "assumed answered." If it is not answered feel free to continue the discussion.