Recently we had an issue wherein it was reported a high call volume on the CUBE . It was suggested to enable the Telephony Denial of Service (TDos).
The SIP stack authenticates the source IP address of an incoming SIP request and blocks the response if the source IP address does not match any IP address in the trusted IP address list. The Trusted IP address needs to be enabled to discard the address that doesn't match.
Below are the commands needs to be configured in CUBE
#conf t
#voice service voip
#ip address trusted authenticate
#allow-connections sip to sip
#sip
#silent-discard untrusted
Also refer the below link for more info.
Source: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube_proto/configuration/15-mt/cube-proto-15-mt-book/voi-cube-tdos-attack-mitigation.html