VoIP glossary
What is Erlang?
An Erlang is a unit of telephone traffic equal to one line busy continuously for one hour, used to work out how many channels a phone system needs.
Erlang explained
Traffic in Erlangs is calls per hour multiplied by average call length in hours. Fifty calls an hour lasting three minutes each is 50 × 0.05 = 2.5 Erlangs. The Erlang B formula then tells you how many channels you need for a given chance of a caller getting a busy signal, known as the grade of service.
Size for the busiest hour, not the daily average. A trunk that is fine at 10 a.m. can still turn callers away at a Monday-morning peak. Channels on a SIP trunk are easy to add, but it helps to know roughly where you need to be before you set a concurrency limit.
Example
A team handles 120 calls in its busiest hour, averaging 4 minutes. Traffic = 120 × 4 ÷ 60 = 8 Erlangs. At 1% blocking, Erlang B suggests about 15 channels.
Put it into practice
Prepaid SIP trunks and wholesale routes from $200. Ask sales for rates or the NOC for a technical answer.