To prevent the toll fraud using the Transfer Rules on Cisco Unity Connection, we can do it either through the Cisco Unity Connection with the Restriction Tables of on Cisco Unified Communication Manager.
On CUCM, I read per documentation that we need to modify the rerouting CSS in the SIP Trunk to include only the required partition, so it should not include the partition of the route pattern to PSTN International numbers.
I would like to say that it depends, because even the inbound CSS in the SIP Trunk must be taken into consideration, it depends to the Transfer Type you select in the Transfer Rules, the reason is:
Release to Switch: this type of transfer uses the SIP Refer method to reroute the call to the specified extension, it is sent in dialog in other words in the existing SIP conversation—> we need a Rerouting CSS in the SIP Trunk between CUCM and CUC.
Supervise Transfer: This type of transfer uses the SIP INVITE method, it is sent out of dialog, this means it’s a new SIP conversation—-> therefore we need the Inbound CSS.
Demystify and Simplify 