adding to the discussion about ACHT. 3 mins always used to be the safe but dumb figure – but there are three factors, nowadays which will undermine that assumption.
Are these connections carring dial up to ISPs. If the answer is yes 3 mins may be too short.
Do the terminating numbers have voicemail. If yes the number of calls answered will obviously be higher but the call durations will be lower. I have done real measurements on 60 trunk systems and found the ACHT to be less than 40 seconds when Vmail is in place.
The last factor is mobiles – which nearly always have Vmail. Vmails as origin or destination will also depress the ACHT.
Hope this helps.
(NB a tip in dimensioning systems) – always model them as slightly under capacity. Traffic very rarely rises in a steep curve. Then look at your growth to see when you are likely to run out and plan to meet it. If you over provide its very difficult to know how much to cut back as its very difficult to measure headroom.