One of the main reasons for this is that the SmartNode does not allow for any "untrusted" host IP Addresses to pass through whenever the "trust" command is configured in the SIP interface.

Managing trusted hosts:

A list of trusted remote peers can be configured on SIP interfaces. If configured, only connections with peers in that list will be accepted. The list may contain IP-addresses or FQDNs or "remote".  If no trusted peers are configured all connections are accepted.

The SmartNode responds with a SIP "503 Service Unavailable” to requests received from hosts which are not trusted. 

interface sip IF_SIP
bind context sip-gateway GW_SIP
route call dest-interface IF_FXO
remote 10.10.10.10 5060
local 10.10.10.10 5060
trust remote

In the example above, only requests coming from the IP address 10.10.10.10 will be accepted.  Anything else will receive the "503 Service Unavailable" from the SmartNode.

In some cases, it is desirable not to send an answer back to an untrusted SIP host. For those cases, in the context sip-gateway, this behavior can be changed. By default the SmartNode responds with a “503 Service Unavailable” to SIP requests received from hosts which are not trusted. With the [no] form of the command below, the SmartNode will not send any message to untrusted hosts.

[node](sip-gw)[name]# [no] answer-untrusted-hosts

context sip-gateway GW_SIP_0

interface LAN
bind interface IF_IP_LAN context router port 5060

context sip-gateway GW_SIP_0
bind location-service LOC_SVC
no answer-untrusted-hosts
no shutdown

The SmartNode will no longer respond to untrusted hosts when the "no answer-untrusted-hosts" command is configured.