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Trunk sizing converts busy-hour call volume and duration into erlangs, then searches for the smallest channel count meeting the target blocking probability under Erlang B. Utilisation rises with group size — a large trunk group carries traffic far more efficiently than several small ones, which is the trunking-efficiency argument for centralised SIP trunks.
Trunk sizing
traffic = calls x duration / 3600; smallest N where ErlangB(traffic, N) <= target; bandwidth = N x per-call rate
One percent blocking in the busy hour is the usual commercial target; 0.1 percent is used for emergency and critical services.
Trunking efficiency. Splitting 20 erlangs across two groups of ten needs more total channels than one group carrying all of it, because each group must carry its own peak.