Work out busy hour call attempts instantly with clear inputs, formula shown and shareable results.
Busy hour call attempts is the standard capacity metric for call servers, because signalling processing scales with attempt rate rather than with talk time. Erlangs measure the media path load instead. Both matter: a system can have ample trunk capacity yet be limited by the SIP proxy's attempts-per-second ceiling.
BHCA
BHCA = subscribers x calls per day x busy-hour share; erlangs = BHCA x average duration / 3600; cps = BHCA / 3600
Because call setup and teardown is the expensive processing step. A short call costs almost as much signalling as a long one, so attempts drive CPU load.
Around 0.03 to 0.10 erlangs per business extension in the busy hour, meaning each extension is on a call 3 to 10 percent of the time.