A phone call is a tiny amount of data
An uncompressed VoIP call using the standard G.711 codec runs at about 87 kbps once you add the packet overhead — call it 0.1 Mbps in each direction. That's roughly a hundredth of what a single HD video stream uses. Not per office. Per call.
Put differently: if a member of staff can watch a YouTube video without it stalling, your line can carry somewhere around ten simultaneous phone calls.
What you actually need, per concurrent call
"Concurrent" is the word that matters. Ten staff who are rarely all on the phone at once don't need ten calls' worth of bandwidth. Size for your realistic peak.
| People on the phone at once | Upload and download needed | Data per hour |
|---|---|---|
| 1 (sole trader) | 0.1 Mbps | ~30 MB |
| 3 concurrent calls | 0.3 Mbps | ~90 MB |
| 5 concurrent calls | 0.5 Mbps | ~150 MB |
| 10 concurrent calls | 1.0 Mbps | ~300 MB |
| 20 concurrent calls | 2.0 Mbps | ~600 MB |
Figures are for G.711 including IP overhead. The compressed G.729 codec uses roughly a third as much, at a small cost in audio quality.
Upload is the number to check, not download
Broadband is sold on download speed, and calls need the same amount going out as coming in. On an older ADSL line you might have 40 Mbps down and 2 Mbps up — that upload figure is your real ceiling, and it still covers about twenty calls. The problem isn't usually capacity; it's what else is competing for it. A big cloud backup running at 9am will do more damage to your call quality than a slow line ever will.
The thing that actually causes bad calls
Voice is a real-time stream, so it cares about consistency far more than volume. Three measurements decide how a call sounds:
- Jitter — variation in packet arrival times. Under 20 ms is comfortable; above 30 ms you get the choppy, robotic effect people blame on "VoIP".
- Packet loss — under 1% is fine and inaudible; above 3% produces clipped words and dropouts.
- Latency — under 150 ms one way. Beyond that people start talking over each other.
A gigabit line with poor jitter sounds worse than a modest one with good jitter. This is why "just get fibre" is such unhelpful advice — and why upgrading your broadband sometimes changes nothing at all.
Test your line in five minutes
- Run a speed test that reports jitter and packet loss, not just download speed.
- Do it during your busiest hour, not first thing on a quiet morning.
- Multiply your realistic peak concurrent calls by 0.1 Mbps and compare against your upload.
- Repeat over wifi from wherever people actually sit — weak wifi causes far more bad calls than weak broadband.
If you'd rather not, send us your postcode and current broadband package and we'll tell you plainly whether it will carry your calls. If it won't, we'll say so rather than sell you a phone system that will disappoint you.
Simple fixes that beat upgrading your broadband
- Put phones and computers on separate wifi bands, or use a wired handset for the busiest desk.
- Enable QoS on the router so voice traffic is prioritised over backups and downloads.
- Move heavy backups and updates to overnight.
- Replace an old router before replacing a line — it's often the actual culprit.
- Set mobile failover so an outage doesn't take the number down at all.
Common questions
Around 100 kbps in each direction per simultaneous call using the standard G.711 codec, including packet overhead — roughly 0.1 Mbps. Five people on the phone at once need about 0.5 Mbps up and down. The compressed G.729 codec needs about a third of that.
Related reading
More on VoIP phone systems or how to choose a provider.