We have Elastix 1.5.2-2 on the local network behind a Smoothwall Advanced Firewall/Gateway with 13 GXP-2000 phones which work well normally. We are sorting out some issues with the Firewall and have been asked by our support people to set the GXP-2000 phones to use "Nat Traversal (Stun)->Yes". We have tested the network and stun info is as follows; Nat with Independend Mapping and Port Dependent Filter - VoIP will work with STUN Preserves port number Does not supports hairpin of media Public IP address: xxx.xxx.xxx.xx The problem is that when we change the GXP-2000s to Nat Traversal "Yes" any calls put through from reception to a phone automatically go to voicemail on the phones. If the settings are changed back to "no or no but send keep-alive" the calls go through normally to the phone. The phones can make calls normally and external users can dial them directly, the issue only appears when calls go through the IVR. Can anyone tell me what I am doing wrong, or what I need to do to get the calls received normally on the phones and not sent to voicemail. I dont know if this is an Asterisk issue or a Grandstream issue?