How Many SIP Trunks Do I Need?
Size your SIP trunk channels from busy-hour call volume or headcount using Erlang B.
One channel carries one simultaneous call. SIP trunk sizing depends on how many calls occur during the busiest hour of your busiest day and the level of blocking you are willing to accept. Enter those values to calculate the number of channels you need.
What happens at other blocking targets
Lower blocking targets require more channels, but the increase is not linear. Moving from 5% to 1% usually requires a relatively small increase in capacity, while moving from 1% to 0.1% requires more channels for a smaller improvement. Use the table below to compare the channel requirement at each blocking target.
| Blocking target | Channels needed | Meaning in practice |
|---|
How the calculation works
Erlang B was developed for traditional telephony and applies directly to SIP channel sizing. It uses your offered traffic and blocking target to calculate the number of channels required.
Erlang B assumes that a caller who encounters a busy condition does not immediately retry. In practice, some callers redial, which can increase traffic above the modeled load. Treat the calculated channel count as a minimum and leave appropriate headroom.
Use your busiest hour for sizing. Use call volume from the busiest hour of a normal busy day. Dividing monthly minutes by working hours smooths out traffic peaks and can understate the capacity you need. If your business has predictable seasonal spikes, include those periods in your sizing.
What the formula does not account for
Erlang B assumes independent call arrivals under steady traffic conditions. Bursty traffic, shared inbound and outbound capacity, failover requirements, and non-call channel use can all increase the number of channels you need.
- Bursty, correlated traffic does not follow Erlang B’s assumption of independent call arrivals. Outbound campaigns, marketing sends, and outage notifications can create sudden traffic spikes. Size those events separately from normal call traffic.
- Inbound and outbound share the pool on most SIP trunks, so a burst of outbound calls eats the channels your inbound callers need. If both directions matter to you, size against the combined peak.
- Failover capacity depends on how much traffic each carrier needs to support during an outage. If either carrier must be able to carry the full load on its own, size each one for the full channel count.
- Non-call channel use also consumes capacity. Fax lines, alarm circuits, paging, and recording legs can occupy channels even when they do not appear in a call-volume report.
Confirm how your provider counts channels and sessions. Providers may define channels, concurrent calls, and sessions differently, and some count each leg of a transferred or recorded call as a separate session. Confirm the billing and session-counting rules when comparing quotes. Apply the same check to any SBC in the path, whose session capacity must support the channel count calculated here.
Frequently asked
Bandwidth is the next step after channel count
Once you know how many concurrent calls your trunks need to carry, the bandwidth required per call depends on the codec, packetization interval, and transport overhead. Actual bandwidth includes both the codec payload and per-packet overhead, so the difference between G.711 and G.729 goes beyond their codec bit rates.
For the complete calculation, see our VoIP bandwidth calculator. For how ProSBC negotiates codecs and manages trunks at the network edge, see the VoIP codec guide.
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