Ask a provider how much bandwidth you need for VoIP and you will get a reassuring answer with a comfortable margin built in. Ask why calls sound bad on a connection that meets that number and the conversation gets vaguer.
The reason is that bandwidth is rarely the problem. Voice traffic is tiny by modern standards. What breaks calls is a different property of the connection entirely.
The actual numbers
A single VoIP call using the common G.711 codec consumes roughly 85 to 100 kilobits per second in each direction once you account for packet overhead. Using the more efficient G.729 codec, that drops to somewhere around 30 kilobits per second.
So ten simultaneous calls on G.711 need about one megabit per second in each direction. Twenty-five calls need roughly two and a half.
| Concurrent calls | Approx. bandwidth each way (G.711) | Practical office size |
|---|---|---|
| 4 | ~0.4 Mbps | Up to about 10 staff |
| 10 | ~1 Mbps | About 20 to 25 staff |
| 20 | ~2 Mbps | About 50 staff |
| 40 | ~4 Mbps | About 100 staff |
Note the ratio of staff to concurrent calls. In a typical office, only twenty to forty percent of extensions are in use at the same time even at the busiest hour. A call center is the exception and should be sized closer to one call per seat.
The practical conclusion is that almost any modern business internet connection has enough raw capacity. If you have a 100 megabit connection and twelve employees, bandwidth is not your problem.
What actually determines call quality
Three measurements matter far more than speed, and none of them appear on the sales page.
Latency
The one-way delay between speaking and being heard. Below 150 milliseconds is fine. Between 150 and 300 the delay becomes perceptible and people start talking over each other. Above 300 the conversation develops that satellite-call rhythm where both parties wait, then both speak at once.
Jitter
Variation in the arrival time of packets. Voice packets are sent at even intervals; if they arrive unevenly, the receiving device has to guess how to reassemble them. Under 30 milliseconds is acceptable, and under 20 is comfortable. Jitter is the most common cause of choppy, robotic audio, and it is entirely independent of how fast your connection is.
Packet loss
Packets that never arrive. Voice has no time to request retransmission, so a lost packet is simply a missing fragment of speech. Under one percent is generally imperceptible. Above three percent, calls become genuinely difficult.
Why calls degrade at specific times of day
If audio is fine at nine in the morning and terrible at two in the afternoon, you are almost certainly looking at contention rather than capacity.
The usual culprits are predictable. Cloud backups scheduled during business hours. Automatic operating system updates rolling out to twenty machines at once. Video conferencing, which consumes twenty to fifty times what a voice call does. Large file transfers. Security camera systems uploading footage continuously. And, on shared cable connections, your neighbors, whose usage peaks at the same time yours does.
Upload capacity deserves particular attention. Cable and DSL connections are asymmetric, frequently offering a tenth of the download speed on the upload side. A connection advertised at 200 down and 10 up has plenty of headroom for voice on paper, but a single cloud backup can saturate that 10 and take every call with it.
Quality of Service: the fix nobody enables
Quality of Service is a router feature that tells your network to prioritize voice traffic over everything else. When the connection is congested, voice packets go first and the file transfer waits a few milliseconds. Nobody notices the file being slower; everybody notices the call being broken.
This is the single highest-value configuration change available for VoIP, it is free, and it is disabled by default on most equipment.
Two caveats. Quality of Service only works within networks you control, so it manages congestion on your own connection but has no effect once packets leave for the wider internet. And router implementations vary considerably in quality — a business-class router does this well, while an inexpensive consumer unit may offer a setting that does very little.
Measuring your own connection
A standard speed test tells you almost nothing useful here. It measures peak throughput for a few seconds under ideal conditions, which is not what voice cares about.
Run a dedicated VoIP quality test instead. Most providers offer one, and several independent tools exist. These simulate actual call traffic and report latency, jitter, and packet loss rather than raw speed.
Test properly to get a meaningful answer: run it at your busiest hour rather than at lunchtime, run it several times across a few days, and run it from a wired connection first to establish a baseline before testing over wireless.
Fixing common problems
- Put desk phones on wired connections. Wireless introduces jitter under load and is the first thing to eliminate when troubleshooting. If a phone must be wireless, put it on a dedicated network and a modern access point.
- Enable Quality of Service on the router. Prioritize by the traffic type or by the phones’ addresses, whichever your equipment supports.
- Move backups and updates outside business hours. Free, and frequently solves the entire problem on its own.
- Check your upload capacity specifically. It is the constraint far more often than download.
- Replace an aging router. Consumer routers handling many simultaneous connections run out of processing capacity long before they run out of bandwidth.
- Separate voice onto its own network segment. A VLAN keeps voice traffic isolated from general office traffic and makes prioritization far more reliable.
- Add a second connection for failover. An inexpensive secondary circuit from a different provider, with a router that can switch automatically, turns an outage into a brief interruption.
What to ask before you sign
When you are evaluating an internet connection for a business that runs on its phones, ask about the upload speed rather than the download, whether the circuit is dedicated or shared with other subscribers, what the service level agreement actually guarantees, and what the typical latency to major destinations looks like.
A modest dedicated connection with low jitter beats a very fast shared one for voice, every time. The headline speed is the least important number on the page.