1. Check the dial plan to find out if the calls between the sites are long distance or international.
2. If the calls are long distance, then you need to create only 1 AAR group with the prefix of 91. If the calls are international, then you would need to create a AAR group for each site.
3. Check the dial plan to see if the calls to the other site are using the local gateway only. If yes, then you can reuse the route pattern. Also you can reuse one of the existing CSS that has access to that route pattern as the AAR CSS. If no, then create a route pattern to specifically match the AAR calls and add it to the long distance or international partition for that site.