We can configure restriction to block external-to-external (OffNet to OffNet) call transfers. You should be careful when using this with Automated Alternate Routing (AAR) as what is usually an OnNet call can become and OffNet call and be dropped.

Route pattern are typically used to restrict outbound call and gateways are used to restrict incoming call.

Configuration Call Transfer Restriction

1. Defining OnNet or OffNet Call Classification on Route Patterns, Intercluster trunks and Gateways
2. Set Block OffNet to OffNet Transfer under clusterwide CallManager Service setting (Service > Service Parameters > Cisco Call Manager)