1. Channels (Erlang B)
Enter your busy-hour traffic. The result is the number of simultaneous calls the trunk must carry so that no more than the chosen share of callers hear a busy signal.
2. Bandwidth
Concurrent calls times the per-call bandwidth of the codec, in each direction. The calls figure is pre-filled from step 1.
3. Summary
The usage estimate is outbound minutes at $0.010/min; inbound is $0.009/min to local numbers. Phone numbers and E911 are extra and there is no per-channel fee, so the channel count changes your bandwidth plan, not your bill.
How the Erlang B formula works
The Erlang B formula is the standard way to size trunk lines: given a traffic load and a number of channels, it gives the probability that a new call finds every channel busy. Named after A. K. Erlang, it has been used in telephone engineering since the early twentieth century.
B(n, A) = (Aⁿ / n!) ÷ Σ(k=0..n) (Aᵏ / k!)
B = blocking probability n = channels A = traffic in ErlangsTraffic in Erlangs is calls per hour multiplied by the average call length in hours. Thirty calls per hour at three minutes each is 30 × (3 ÷ 60) = 1.5 Erlangs.
Worked example
- Traffic: 30 calls/hour × 3 minutes ÷ 60 = 1.5 Erlangs
- For 1% blocking: 6 channels (5 channels would block 1.4% of calls)
- With 20% headroom: 8 channels
Erlang B assumes blocked calls are cleared (the caller does not retry at once), which fits how people use business phones. For the planning side, read SIP trunk capacity planning.
Codec bandwidth
Each codec compresses audio differently. The "with overhead" column adds the 40 bytes of IP, UDP and RTP headers on every 20 ms packet.
| Codec | Payload | With overhead | Quality | Notes |
|---|---|---|---|---|
| G.711 µ-law / A-law | 64 kbit/s | 87.2 kbit/s | Excellent | Default on IPComms trunks; no licensing |
| G.729 | 8 kbit/s | 31.2 kbit/s | Good | Constrained links; licensed on some PBXs |
| G.722 | 64 kbit/s | 87.2 kbit/s | Wideband | Internal calls only; the public network is narrowband |
| Opus | 6–510 kbit/s | ~40 kbit/s | Excellent | WebRTC and modern endpoints, not carried on trunks |
| T.38 fax | Variable | ~40 kbit/s | n/a | Fax relay |
Common sizing scenarios
10-person office
- Traffic: about 0.5 Erlangs
- Channels: 3–4
- Bandwidth (G.711): 0.35 Mbit/s
- Outbound usage: about $9/month
50-seat contact center
- Traffic: about 25 Erlangs
- Channels: 35–40
- Bandwidth (G.711): 3.5 Mbit/s
- Outbound usage: about $450/month
200-employee company
- Traffic: about 10 Erlangs
- Channels: 18–22
- Bandwidth (G.711): 1.9 Mbit/s
- Outbound usage: about $180/month
The contact center assumes half the agents on a call at once (manual, human-initiated calling; automated dialing is not permitted on the network). The 200-employee case assumes about 10% peak utilization. See also SIP trunking vs. PRI.
QoS for SIP trunking
DSCP markings
- SIP signaling: DSCP 24 (CS3)
- RTP media: DSCP 46 (EF, expedited forwarding)
Network targets
- Latency: under 150 ms one way, ideally under 80 ms
- Jitter: under 30 ms, with a 40–80 ms jitter buffer
- Packet loss: under 1%, ideally under 0.1%
- Dedicated bandwidth: reserve voice bandwidth separately from data
Jitter buffer
- Minimum 20–40 ms, maximum 80–160 ms, adaptive mode
- On high-jitter links raise the maximum to 200 ms, accepting the added delay
Questions
How many SIP channels do I need for 50 employees?
A typical 50-person office needs about 8–12 concurrent channels, assuming 2–3 calls per employee per hour at peak and a 3-minute average call. Use the calculator with your own figures; Erlang B at 1% blocking gives the exact number.
What bandwidth does a SIP trunk need?
Between 31.2 kbit/s (G.729) and 87.2 kbit/s (G.711) per call in each direction, including overhead. Ten concurrent G.711 calls need about 1.74 Mbit/s each way. Add a 20% buffer and keep latency under 150 ms and jitter under 30 ms.
Does IPComms limit concurrent channels?
There is no per-channel fee and no set channel count. New accounts start with a fraud-protection limit that support raises on request, so tell us your expected peak. Beyond that, capacity is set by your bandwidth and your PBX.
What codec should I use?
G.711 µ-law for North America: best quality, no compression artifacts, no licensing. G.729 when bandwidth is tight; it uses about a third as much but needs a license on some platforms. IPComms trunks carry G.711 µ-law, G.711 A-law and G.729.