Cisco has stepped up the toll-fraud prevention in IOS 1.1(2)T and later by rejecting all calls sourced from non-trusted IP addresses by default. This means all inbound calls will fail until the source addresses are added to the trusted listed of addresses.
The trusted IP addresses are configured under Voice Service Voip parameters as shown below:
voice service voip
ip address trusted list
ipv4 203.0.113.100 255.255.255.255
ipv4 192.0.2.0 255.255.255.0
More info can be found here:
http://www.cisco.com/en/US/tech/tk652/tk90/technologies_tech_note09186a0080b3e123.shtml
