SIP OPTIONS between two servers communicating but trunk down

Hi guys,

I was troubleshooting communication between our SBC’s and our asterisk appliance at a remote client location.
SIP options were being sent from our appliance to our SBC. The SBC replied with 200 ok. I assumed the trunks must have been up since the SBC SIP traffic was received by the asterisk applicance. Yet when we ran asterisk -rx ‘show sip peers’ on our appliance the trunks to our SBC’s were unavailable. On the asterisk appliance, my manager noticed by examining the SIP traffic being received from the SBC that an interface was being used that the client later confirmed was no longer in service. The customer fixed something with their firewall NAT then asterisk sip show peers showed the trunks to our SBC’s as up. How could SIP traffic from the SBC’s be received by the appliance but the trunk to the SBC’s be down on the appliance if there was a NAT issue? Shouldn’t the SIP traffic from the SBC to the appliance not have been received if there was a NAT issue?