NAT Specialist for SIP Audio Issue

I'm experiencing a one-way audio issue with my SIP setup. I have a GOIP Gateway configured to forward calls to FreePBX/Asterisk. The setup works, but because devices are behind NAT, audio only travels one way.

Your expertise is needed to investigate and advise on resolving this issue.

Key points:
- One-way audio issue
- SIP phone is behind NAT
- Potential loopback address issue (127.0.0.1)
- Access to NAT router/firewall settings can be arranged

Ideal Skills & Experience:
- Expertise in NAT traversal
- Familiarity with SIP and RTP protocols
- Experience with FreePBX/Asterisk configurations
- Network troubleshooting skills

Please provide a detailed approach and estimated time for resolution.