The perpetual surprise of VoIP planning: a call needs almost nothing. A G.711 call, the uncompressed standard, uses 64 kbps of audio, roughly 87 to 100 kbps per direction once packet overhead is counted. Compressed codecs use less. Against a 100 Mbps business connection, a phone call is a rounding error, which is exactly why "add more bandwidth" so rarely fixes call problems.
The actual sizing math
Count concurrent calls, not employees. A 30-person office rarely exceeds 8 to 10 simultaneous calls; at ~100 kbps each, that is about 1 Mbps of voice, both directions. Even generously doubled for headroom, any functioning business connection has the capacity. The formula worth keeping: peak concurrent calls x 100 kbps, compared against your upload speed, because upload is where business connections are thin and where your outgoing voice lives.
Why upload is the side that bites
Asymmetric connections (cable, DSL, some fixed wireless) advertise the download number: "500 by 20" means 20 Mbps up. Your outgoing audio, screen shares, cloud backups, and camera uploads all compete for that 20. When a backup saturates the upload, your voice packets queue behind it, and the people on the far end hear you break up while you hear them fine, a one-directional complaint that confuses everyone and is solved by QoS, not by more download.
What actually causes 'not enough bandwidth' symptoms
- Congestion at peak moments: the 2 PM sync, not the average load. Averages lie; voice lives in the worst 20 milliseconds.
- Bufferbloat: routers that queue deeply under load add hundreds of milliseconds of latency while speed tests still look great.
- Wi-Fi contention: airtime, not bandwidth. A busy 2.4 GHz band mangles voice with retransmissions at any speed tier.
- The guest network: visitors streaming on the same circuit as your phones, ungoverned.
Sizing guidance you can actually use
For voice alone, almost any dedicated connection works. The honest requirements are: enough upload that peak calls plus normal business traffic fit with room to spare, a router that does QoS properly, and wired phones. If a connection cannot manage that, the fix is usually configuration or a better router before it is a bigger circuit, and the call quality guide walks the diagnosis in order.
Rule of thumb: 100 kbps per concurrent call, measured against upload, at the busiest moment of your day. If that math fails on a modern business connection, the problem is management, not capacity.
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