The sipXcom proxy includes built-in compensation for both near and far end NATs (sipXrelay, also known as Media Relay), and sipXbridge (handles trunking) provides what you need to connect to external SIP Trunk services.
If you are really serious about SIP trunking and NAT traversal an external SBC is strongly recommended.
The aforementioned are known to work but you may want to trial with your installation. They have a few limitations:
The following SBC's are known to work to varying degrees with sipXcom. Try before you buy (if you can).