For NAT traversal you must rely on your SIP service provider. In general, every SIP service provides relaying for both signaling and media, which takes care of the NAT issue. This means that no special settings are necessary in the client.
For Audio Calls you can enable the use of ICE negotiation in Advanced->NAT Traversal->Use ICE. ICE requires the presence of STUN servers, they must be provided by the SIP service provider.
The ICE negotiation details are displayed in the "RTP Logs window". While the ICE negotiation progresses, its status is also displayed in the Audio Call information, for example the gathering of ICE the candidates and the probing of the remote candidates. t
Once the audio stream is established, the audio information area is updated with the negotiated codec type and the sample rate. The active RTP end-points and the ICE candidates types are displayed while hovering the mouse over the audio status information area.
IM and File Transfers
To traverse the NAT for MSRP related sessions (Instant Messaging, File Transfer) a MSRP relay configured for the called party domain is needed. The SIP service provider that provides the SIP Account must support this feature (e.g. http://sip2sip.info).