Troubleshooting One-Way Audio in VoIP – FreePBX, 3CX, SIP, RTP & Port Forwarding
One-way audio (you can hear the other person but they can’t hear you — or vice versa) is one of the most common and frustrating issues when running a self-hosted PBX. It happens far more often than full call failure.
Why One-Way Audio Happens
The Root Cause
SIP vs RTP
SIP (ports 5060/5061) only handles call setup. RTP carries the actual voice and uses a wide range of UDP ports — this is where most problems occur.
FreePBX / Asterisk
Default RTP Range
10000 – 20000 UDP
3CX
Default RTP Range
9000 – 9500 UDP (sometimes wider)
Most Effective Fixes (Ranked)
1. Best Long-Term Solution
Use a VPN (WireGuard or Tailscale recommended). This eliminates most NAT and RTP problems entirely.
2. Proper Port Forwarding
Forward the correct RTP range on your router and make sure the PBX is set to use the public IP (not local).
3. Critical Router Settings
Disable SIP ALG immediately • Enable Consistent NAT • Consider enabling STUN / ICE on extensions.
4. Quick Wins
Use 3CX Tunnel (port 5090) • Set phones to use the PBX public IP • Check for double NAT.
Diagnostic Checklist
Quick Checks
Ask These Questions
Is it happening on all external calls? Only mobile apps? Internal calls work fine? Does it happen in both directions?
Test Method
Easy Test
Call a public echo test service (e.g. 4443 on many providers) and a mobile phone. This helps isolate the problem quickly.
When to Get Help
Call for Backup
Double NAT, CGNAT, complex firewall rules, or one-way audio on all external calls after trying the basics.
What’s Your Setup?
Help Us Help You
Are you running FreePBX, 3CX, or another system? Is the issue happening on internal calls, external calls, or both? What router are you using?
Share Details
PBX version, router/firewall model, and exact symptoms help the community give accurate advice fast.
One-way audio is almost always solvable once you correctly identify whether it’s a port forwarding, NAT, or RTP configuration issue.
Disclaimer
This content is for educational and informational purposes only. It is not technical support. Always back up your PBX configuration before making changes. Network and VoIP setups can vary greatly depending on your ISP and router.
No replies yet. Be the first to join the discussion!
