Ever had a call connect successfully, but only one side could hear anything?

For example, you can hear the customer, but the customer can’t hear you. Or the other way around.

If so, you’re dealing with One-Way Audio. Don’t worry, this is a fairly common issue on VoIP systems like 3CX and can usually be fixed without reinstalling anything.

Let’s find out what causes it and how to fix it!

What Is One-Way Audio? #

One-Way Audio is a condition where a call connects successfully, but audio can only be heard in one direction.

For example:

You can hear the other person but they can’t hear you, or the reverse.

This issue is almost always related to network settings, especially NAT, the firewall, or STUN.

Causes of One-Way Audio #

Some of the most common causes include:

1. NAT Not Configured Correctly #

NAT (Network Address Translation) connects a local network to the internet.

If NAT is misconfigured, voice packets (RTP) can fail to reach their destination.

The result:

• The call stays connected.
• But audio only travels in one direction.

2. Firewall Blocking RTP Ports #

An overly strict firewall can block the audio path.

Usually the call signaling (SIP) still gets through, but the voice data (RTP) gets held up.

The result?

The call connects, but the audio has problems.

3. Incorrect STUN Configuration #

If you’re using STUN for remote devices, an incorrect or inactive STUN server can prevent the audio path from being established.

4. A Port Hasn’t Been Opened #

The RTP port needs to be reachable from both sides.

If a port is closed, audio can be lost in one direction.

How to Fix One-Way Audio #

Follow the steps below.

1. Run the 3CX Firewall Checker #

Go into the Admin Console and run the Firewall Checker.

This tool checks whether all the required ports are open.

If it shows a Failed status, some network configuration needs fixing.

2. Check the NAT Router #

Make sure the router is using the correct NAT configuration.

Things to check:

• Port forwarding is set up correctly.
• There’s no double NAT.
• The router isn’t blocking RTP.

If you’re using two routers at once, try switching to a single router or enabling bridge mode.

3. Make Sure the RTP Port Is Open #

In general, the RTP port must be able to pass through the firewall.

If this port is blocked, audio won’t be able to travel normally.

Make sure your firewall rules allow RTP traffic matching your 3CX server’s configuration.

4. Check the STUN Configuration #

If you’re using an IP Phone or softphone from outside the office:

• Make sure the STUN address is correct.
• Use a STUN server that’s still active.
• Avoid unnecessary STUN configuration if you’re already using an SBC or a tunnel.

5. Restart the Device After Changing the Configuration #

After making changes:

• Restart the IP Phone.
• Restart the 3CX app.
• Make a test call.

Sometimes network changes only take effect after the device reconnects.

How to Confirm the Problem Is Solved #

Make a test call.

Everything is fine if:

• You can hear the other person.
• The other person can also hear you.
• There’s no choppy or dropped audio.

If One-Way Audio still happens, check the Firewall Checker results and your network configuration again.

Tips to Prevent One-Way Audio from Happening Again #

To keep this problem from coming back, do the following:

• Always run the Firewall Checker after changing your network.
• Avoid using double NAT.
• Make sure the RTP port isn’t blocked by the firewall.
• Use a router that supports VoIP.
• Check the STUN configuration if you’re using remote devices.

Conclusion #

One-Way Audio can definitely disrupt communication, but the cause is usually something fairly simple: network configuration.

By making sure NAT, the firewall, STUN, and the RTP port are all configured correctly, most One-Way Audio cases can be resolved quickly. If the problem persists after all these steps, check your network logs or contact your system administrator so the configuration can be reviewed further.

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