Telephone System Sizing Calculator
PBX trunk and extension count from user and traffic data.
Inputs
%
%
Results
Required trunk lines
17
Extensions needed115
Busy hour traffic (Erlangs)10
Actual blocking (%)0.71
External (PSTN) trunks12
SIP channels (VoIP)17
Bandwidth G.711 (Mbps)1.48
Bandwidth G.729 (Mbps)0.54
Busy hour call volume200
System tier (1-4)2
Internal Capacity30
How to Use This Calculator
- Enter the number of employees and the percentage who are simultaneous callers.
- Set the average call duration in minutes and the calls per hour per user.
- Input the desired blocking probability (standard is 2% or less using Erlang B).
- Review the Traffic Intensity in Erlangs and the Number of Lines Required.
- Add 20% capacity headroom over the calculated lines to handle traffic spikes.
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