I’d like to add my viewpoint.
According to ITU, there is a relationship between Erlangs and BHC that can be extracted from the following:
A = M * D * H /(120 * E)
Providing that:
A: Conversational Erlangs
M: Monthly minutes of voice traffic
D: Month to day factor (1/22 to 1/30)
H: Day to Hour factor (1/7 to 1/10)
E: Efficiency factor of the trunk (0.9 avg).
It’s being considered an avg. call duration of 120 sec.
If you play with the formula and have in mind a basic relationship
(for non-blocking trunks) that’s:
BHCA = BHC / ASR
Where BHCA: BH Call Attempts
BHC: BH Calls
ASR: Attempting Size Ratio
(bet. 0.65 to 0.8)
you’ll get a relationship between A and BHCA.