One-Way Audio on VoIP Calls: What Causes It and How to Fix It
One-way audio — where one person can hear the other, but not the reverse — is almost always caused by audio data packets failing to travel back to one side of the call, most commonly due to a NAT (Network Address Translation) or firewall issue. This is different from general call quality problems like breaking up or delay, which are usually about bandwidth. One-way audio specifically points to a routing problem, where the call connects but the audio itself doesn't complete the journey in both directions.
Why Is One-Way Audio Different From Other Call Quality Issues?
A VoIP call actually involves two separate things happening at once: signalling, which sets the call up, and media, which carries the actual voice audio using a protocol called RTP. One-way audio happens when signalling works fine — the call connects, rings, and appears to be live — but the RTP audio stream doesn't make it back in one direction. This is why one-way audio often feels confusing: everything looks like it's working, except the sound itself.
Common Cause 1: NAT (Network Address Translation) Issues
Most business networks use NAT to let multiple devices share one public IP address. This works well for most traffic, but VoIP's separate signalling and media streams can confuse NAT devices, particularly older or misconfigured ones, causing the audio stream to get lost on its way back.
Fix: Most modern VoIP systems and routers handle NAT correctly by default, but if one-way audio is a recurring issue, checking your router's NAT and SIP handling settings — or having a specialist check them — is usually the first place to look.
Common Cause 2: SIP ALG Interference on Your Router
SIP ALG (Application Layer Gateway) is a router feature designed to help VoIP traffic pass through NAT more easily. Ironically, it's one of the most common causes of one-way audio, since many router implementations of SIP ALG are poorly designed and actively interfere with the call rather than helping it.
Fix: Try disabling SIP ALG in your router's settings if it's currently switched on. This single setting resolves a surprising number of one-way audio cases.
Common Cause 3: Firewall Blocking RTP Media Ports
VoIP systems use a specific range of network ports for the RTP audio stream, separate from the ports used for call signalling. If a firewall is blocking or restricting this port range, calls connect but audio doesn't flow properly in one or both directions.
Fix: Check your VoIP provider's documentation for the exact port range required, and confirm your firewall allows this traffic through. This is a common issue after a firewall replacement or a security policy update that wasn't checked against VoIP requirements.
Common Cause 4: Asymmetric Routing
Occasionally, audio takes a different network path in each direction, particularly on more complex networks with multiple internet connections or routing rules. If one path is blocked or misconfigured while the other works fine, audio flows one way but not the other.
Fix: This is a more technical networking issue, usually best diagnosed by someone with visibility of your specific network setup rather than trial and error.
Common Cause 5: One Party's Network, Not Yours
If you're on a call with someone outside your organisation, the cause of one-way audio might sit entirely on their end, not yours — particularly relevant if the problem only happens with specific external contacts rather than internally.
Fix: If one-way audio only ever seems to affect calls with one particular external number or organisation, it's worth mentioning this to them directly, since the fix may need to happen on their network rather than yours.
How to Fix One-Way Audio: A Practical Checklist
- Check whether the issue happens on every call or only some — internal calls only, external only, or specific contacts.
- Disable SIP ALG on your router if it's enabled.
- Confirm your firewall allows the RTP port range your VoIP provider specifies.
- Restart the router and any VoIP hardware, which occasionally clears a stuck NAT mapping.
- If the issue persists across multiple calls and contacts, it's time to get a specialist to check NAT and routing configuration properly.
When to Get a Specialist Involved
One-way audio is one of the VoIP issues most likely to need proper network expertise rather than guesswork, since NAT and firewall configuration aren't always straightforward to diagnose from a router's basic settings page. If disabling SIP ALG and checking firewall ports doesn't resolve it, that's a reasonable point to bring in someone who can look at the network configuration directly.
Frequently Asked Questions
Is one-way audio the same problem as calls breaking up? No. Breaking-up audio is usually a bandwidth or congestion issue affecting quality throughout a call. One-way audio is a routing problem where the audio stream fails to travel in one direction entirely, even though the call itself connects normally. Our guide to why calls break up covers the bandwidth-related causes separately.
Can one-way audio happen on some calls but not others from the same office? Yes, particularly if it's related to a specific external contact's network rather than yours, or if certain call types route differently through your firewall.
Does upgrading my broadband fix one-way audio? Rarely. Since this is usually a routing or NAT configuration issue rather than a bandwidth problem, more bandwidth alone typically doesn't resolve it.
Is SIP ALG something I should just turn off permanently? For most business VoIP setups, yes — SIP ALG causes more problems than it solves in practice. If you're unsure, it's worth checking with your VoIP provider before changing router-wide settings.
One-way audio is fixable, but it usually needs the right diagnosis rather than trial and error. Request a quote if you'd like T2K to check your network configuration properly, or read our guide to why VoIP calls break up if your issue sounds more like general call quality than one-way audio specifically.
