The following is quoted from the link above. I am not quite follow what it really means. Why force on-net calls dialed as PSTN?
Design Guidelines for Multisite Deployments
You can force on-net calls dialed as PSTN for destinations within the cluster by adding translation patterns that match the E.164 DID ranges for each site and that manipulate the digits so they match the destination's internal extension. However, remember to provision AAR accordingly, using one of the following methods, so that automated PSTN failover is possible when the IP WAN is out of bandwidth:
?Configure the AAR calling search space to exclude the partition containing the "force on-net" translation patterns but to include a partition containing the regular route patterns pointing to the PSTN.
?Configure the AAR group to prefix digits with a special character such as *, and configure additional route patterns such as *9.! and *9.!# (or *0.! and *0.!#) in the regular partition.