Call quality and troubleshooting · For 3CTel customers
Why can't the other person hear me (one-way audio)?
One-way audio means the call set up correctly but the audio stream is reaching only one side. Because voice travels separately from the call signalling, a router or firewall can allow the call to connect while blocking or misdirecting the audio. The usual cause is SIP ALG on the router, followed by double NAT or a firewall rule blocking the audio ports. It is a network setting, not a faulty phone.
Step by step.
Confirm the direction
Note whether you can hear them but they cannot hear you, or the reverse. Check that the phone or headset is not muted and the microphone is selected in the app.
Restart in order
Restart the modem, then the router, then the switch and phones, waiting for each to come online. A stale NAT table can cause one-way audio that clears after a restart.
Test on a different network
Make a call from the mobile app on cellular data. If audio is fine there, the office network is the cause.
Turn off SIP ALG
In the router settings, disable SIP ALG (also labelled SIP Helper or SIP Passthrough). Restart the router and the phones, then test again.
Check for double NAT
If an ISP modem and your own router are both routing, put the modem in bridge mode or ask your IT provider to do so. Two layers of NAT commonly break the audio path.
Check firewall rules
Ask whoever manages the firewall to confirm outbound UDP is allowed and that no rule or intrusion-prevention feature is inspecting or blocking voice (RTP) traffic.
Contact 3CTel
If audio is still one-way, contact support with the time of a test call and the device used. The platform can show whether audio was received from your side.
Why the call connects but audio does not
A VoIP call has two parts. Signalling sets the call up and tears it down; the audio stream carries voice. The signalling path tells each side where to send audio. When a router with SIP ALG rewrites the addresses in that signalling, one side sends audio to the wrong place and the other hears silence. The phone rings, the call is answered, and one person talks into nothing.
The same effect happens when a firewall allows the call to be set up but blocks the return audio, or when two routers each translate addresses and the audio ends up at the wrong one.
Quick checks before changing the router
- Mute button on the phone or headset, and the microphone selection in the desktop app.
- Headset cable or Bluetooth connection: try the handset or another headset.
- Does it happen on every call, or only on some numbers or only internal calls?
- Does it happen on desk phones, apps, or both?
When to contact 3CTel
If the problem started after a new router, firewall, or ISP change, the cause is nearly always on that equipment and support can confirm the settings to ask for. If nothing on your network changed and one-way audio appears suddenly on all devices, contact support so the routing can be checked from the platform side. A 3CTel specialist can walk your IT provider through the router settings.
Related questions.
Can a phone cause one-way audio?
Occasionally, if the handset or headset is faulty or the wrong microphone is selected in the app. If the problem follows the network rather than the device, and stops on cellular data, the network is the cause.
Why does it only happen on some calls?
Intermittent one-way audio usually comes from NAT entries expiring or from calls routed through a different carrier path. Disabling SIP ALG and correcting double NAT normally makes it consistent again.
Do I need to open ports on my firewall?
Normally no inbound ports are required. Phones and apps open connections outward and keep them alive. What matters is that outbound UDP is permitted and nothing on the router inspects or rewrites voice traffic.
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