One-Way Audio on Calls
You can hear them, they can't hear you — or the reverse. It's a network routing problem.
One-way or intermittent audio is one of the clearest signs of a network-layer VoIP problem rather than a broken phone. It happens when voice (RTP) traffic can't travel symmetrically between your network and the far end.
Likely causes
Ranked from most to least common based on what we see in the field.
- Firewall or router blocking RTP traffic asymmetrically
- NAT or SIP ALG rewriting packets incorrectly
- QoS misconfiguration or bandwidth contention on the LAN/WAN
- Codec mismatch between endpoints
- SIP trunk or carrier-side routing issue
- Firmware bug on the handset or gateway
What you can check first
Safe steps that take a few minutes and can save a truck roll.
- Note whether it happens on all calls or only calls to/from certain numbers
- Check if it happens on every handset or just one
- Restart the affected handset to clear a stuck SIP session
- Check your router/firewall for a setting called SIP ALG and note if it's enabled
- Test a call over a different network (e.g., a cellular hotspot) to isolate LAN vs. carrier
How we diagnose it
One-way audio almost always traces to how RTP packets are routed through your firewall or router. We capture and inspect SIP/RTP traffic to see exactly where packets are being dropped or mis-translated.
Fixing it
The fix is usually a firewall or router configuration change — disabling SIP ALG, opening the correct RTP port range, or adjusting NAT settings — combined with QoS tuning so voice traffic isn't competing with other traffic on your network.
- Firewall and NAT configuration review and correction
- QoS and VLAN setup for voice traffic
- Codec and gateway configuration checks
- Carrier-side escalation when the issue is on their network
Frequently asked
Is one-way audio a sign my phones are failing?+
No — it's almost always a network configuration issue, not a hardware failure. The phones themselves are typically fine.
What is SIP ALG and why does it cause this?+
SIP ALG is a router feature meant to help VoIP traffic but frequently breaks it by rewriting packets incorrectly. We recommend disabling it on most business networks.
Does this only happen on VoIP, or can PBX systems have it too?+
It's most common on VoIP and SIP trunk calls, but analog-to-digital gateways can exhibit similar symptoms from misconfigured routing.
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