Call Quality Troubleshooting
VoIP complaints are vague by nature — "the phones are bad today" — but the symptoms map cleanly onto causes once you know the patterns. One-way audio is almost always NAT or firewall. Choppy audio is almost always jitter. Calls dropping at a consistent interval is almost always a timer. Knowing those three saves most of the investigation.
Establish these first
- Internal or external calls? Internal-only problems are your network; external-only points at the link or provider.
- One phone, one area, or everyone?
- All the time, or at particular times? Time-of-day patterns mean congestion.
- What changed? Firmware, a new firewall, a new switch, or a network change.
Start here
| Symptom | Most likely cause |
|---|---|
| One party cannot hear the other | NAT or firewall blocking the media stream |
| Choppy or robotic audio | Jitter or packet loss, usually from congestion |
| Audio delayed, people talk over each other | Latency, often a long path or an overloaded link |
| Calls drop at the same interval every time | SIP session timer or NAT keepalive mismatch |
| Phone will not register | Credentials, provisioning, or network reachability |
| Bad only during business hours | Congestion — voice queued behind other traffic |
| Echo on speakerphone | Acoustic feedback, not the network |
| Only external calls affected | Internet link or provider side |
| Only one desk affected | Cable, switch port, or that handset |
| Phone reboots randomly | PoE budget exceeded on the switch |
| PC behind the phone is slow | 100 megabit pass-through port |
| Quality dropped after a firewall change | SIP ALG re-enabled, or inspection applied to voice |
One-way audio
The call connects and rings, someone answers, and one side hears nothing. Signalling worked; media did not. That narrows it immediately — see how VoIP works for why the two are separate.
- Disable SIP ALG on the router or firewall This is the first thing to try and it resolves a large share of cases. The feature rewrites signalling and frequently gets the media addressing wrong.
- Check firewall rules for the media stream Signalling and audio use different ports. A rule permitting signalling does not necessarily permit the audio.
- Check for double NAT A modem in router mode behind your firewall creates two layers of translation and breaks media addressing. Put the modem in bridge mode.
- Check the provider's recommended settings Some require specific NAT keepalive intervals or their own address ranges permitted.
- Establish direction Which side hears nothing. Inbound-only or outbound-only failure points at which direction of media is blocked.
- Test a phone on a different network If it works on a mobile hotspot, the problem is your network, not the phone or provider.
Before investigating anything else on a network with a general-purpose router or firewall, find the SIP ALG setting and turn it off. It is enabled by default on a great many devices and it is the most common single cause of one-way audio, registration failures, and calls dropping. It was designed to help and it reliably does the opposite.
Choppy or robotic audio
This is jitter or packet loss. Bandwidth is almost never the cause, because voice uses very little of it.
- Check QoS is actually applied on every switch in the path and on the internet link. One device not honouring the marking undoes the chain.
- Look for what else is using the link at the times it is bad. Backups, cloud sync, and update rollouts are the usual culprits, and they typically run to a schedule that matches the complaint.
- Prioritise upstream especially. Upload is usually the slower direction and the one that breaks call quality first.
- Check the phone's own call statistics. Most business phones display jitter, loss, and latency for the current call, which turns a subjective complaint into numbers.
- Check for a duplex mismatch or errors on the switch port. Rising error counters on that port explain a lot.
- Move the phone off Wi-Fi if it is on it. Wireless adds jitter that wired does not.
Most business IP phones expose per-call statistics — jitter, packet loss, and round trip time — either on screen or through the web interface. Reading them during a bad call converts "it sounds terrible" into a measurement, and immediately distinguishes a network problem from an acoustic one.
Calls dropping at a consistent interval
A call that always drops at around the same duration is a timer, not a quality problem.
| Cause | What to check |
|---|---|
| SIP session timer mismatch | The phone and provider disagree on refresh interval, so one considers the call ended. Match the session expiry settings. |
| NAT translation timeout | The firewall closes the mapping mid-call. Increase the UDP timeout or shorten the keepalive interval. |
| Firewall idle timeout | Some firewalls close sessions considered idle. Voice media does not always look active to inspection. |
| Provider-side limit | Rare, but worth ruling out by asking. They can see their side of the call. |
Phones will not register
- Check the phone has an address on the voice VLAN. If it landed on the data VLAN, the switch port is misconfigured.
- Check credentials and provisioning. A single wrong character in an extension password fails silently.
- Check SIP ALG again. It breaks registration as well as audio.
- Check the firewall permits SIP signalling outbound to the provider.
- Check the time on the phone. Wrong time breaks certificate validation where signalling is encrypted.
- Reboot after fixing the network, since phones cache provisioning results and may not retry promptly.
Echo, and other things that are not the network
- Echo is usually acoustic. A speakerphone picking up its own output, or a headset microphone too close to the speaker. Reducing speaker volume normally resolves it.
- Echo on one side only means the far end is causing it, however it sounds to you.
- Background noise is a microphone and environment issue. A noise-cancelling headset helps far more than any network change — see choosing a business headset.
- Volume too low is often the handset or headset gain setting rather than the line.
- Random reboots point at PoE budget rather than the phone. Check the switch.
Voice problems caused by congestion disappear when nothing else is using the network, so testing at 7am proves nothing. If the complaint is time-related, reproduce it at that time — and check what is scheduled to run then. Backups and update rollouts are the most common answer.
Official manufacturer resources
Call statistics displays, SIP settings, and diagnostic tools vary by phone model and platform. Your voice provider's documentation takes precedence for platform-specific settings.
Calls sounding bad and nobody can say why?
Tell us whether it affects internal or external calls, when it happens, and what the phone's call statistics show. That combination usually identifies it — and it is rarely the phones.
Still stuck? Talk to someone who works on this hardware daily.
Tell us the model, what you have already tried, and what you are seeing. We will tell you whether it is a setting, a consumable, or a part — and we will say so if you do not need to buy anything.
