Stop guessing. Here is the exact calculation for how much bandwidth your VoIP system needs, with UK-specific considerations.
The Formula
Required bandwidth = Concurrent calls × Per-call bandwidth × 1.5 safety margin
Step 1: Estimate Concurrent Calls
| Office Size | Typical Concurrent Calls | Peak (Tuesday 10 AM) |
|---|---|---|
| 5 users | 2-3 | 4 |
| 10 users | 4-6 | 8 |
| 25 users | 8-12 | 16 |
| 50 users | 15-25 | 35 |
| 100 users | 30-50 | 70 |
Rule of thumb: Peak concurrent calls = 70% of total users (not 100% — not everyone calls simultaneously).
Step 2: Calculate Per-Call Bandwidth
| Codec | Per Call (Kbps) | Quality | When to Use |
|---|---|---|---|
| Opus | 80 | Excellent | Default choice |
| G.722 | 100 | Very Good | Legacy compatibility |
| G.711 | 100 | Good | Maximum compatibility |
| G.729 | 40 | Acceptable | Very low bandwidth situations |
Use Opus. It gives the best quality at the lowest bandwidth, with built-in error correction.
Step 3: Calculate Total
| Office | Concurrent | Codec | Raw Need | With 1.5x Safety | Recommended Circuit |
|---|---|---|---|---|---|
| 5 users | 4 | Opus | 320 Kbps | 480 Kbps | Any broadband |
| 10 users | 8 | Opus | 640 Kbps | 960 Kbps | 5+ Mbps upload |
| 25 users | 16 | Opus | 1.28 Mbps | 1.92 Mbps | 10+ Mbps upload |
| 50 users | 35 | Opus | 2.8 Mbps | 4.2 Mbps | 20+ Mbps upload |
| 100 users | 70 | Opus | 5.6 Mbps | 8.4 Mbps | 50+ Mbps upload |
Step 4: Check Your Upload (Critical)
VoIP uses symmetrical bandwidth. Download speed is usually fine. Upload is the bottleneck.
| UK Connection Type | Typical Upload | VoIP Users Supported |
|---|---|---|
| FTTC (BT, Sky) | 10-20 Mbps | 10-25 users |
| FTTP (BT Full Fibre) | 50-115 Mbps | 50-150 users |
| Virgin Cable | 20-52 Mbps | 20-65 users |
| Leased Line | Symmetrical | Unlimited |
| 5G Fixed Wireless | 10-30 Mbps | NOT recommended for VoIP |
FTTC warning: If your FTTC upload is 10 Mbps and you have 25 VoIP users consuming 2 Mbps, you have 8 Mbps left for everything else (cloud apps, email, file sharing, video). During peak usage, voice quality will degrade unless QoS is configured.
Step 5: QoS Configuration
Without QoS, a large file upload or video stream steals bandwidth from voice. With QoS, voice always gets priority.
| Traffic | DSCP Value | What Your Router Does |
|---|---|---|
| Voice (RTP) | 46 (EF) | Always gets bandwidth first |
| SIP signalling | 24 (CS3) | Second priority |
| Video | 34 (AF41) | Third priority |
| Everything else | 0 (BE) | Gets whatever is left |
Router configuration (most business routers):
- Log into router admin
- Find QoS or Traffic Management settings
- Create rule: UDP ports 10000-20000 → DSCP 46 → Priority: Highest
- Create rule: TCP/UDP port 5060-5061 → DSCP 24 → Priority: High
- Apply and save
The Quick Answer
| Your Situation | What You Need |
|---|---|
| Under 10 users | Any broadband works, configure QoS |
| 10-25 users | FTTC or better, QoS mandatory |
| 25-50 users | FTTP or business broadband, QoS + voice VLAN |
| 50+ users | Dedicated fibre or leased line |
DialPhone runs a free network readiness assessment before every deployment. Their onboarding team checks your bandwidth, configures QoS, and verifies call quality — included in every plan at no extra cost.
