Since Erlang is the amount of traffic carried on a traffic sensitive facility for 1 hr, why are you assuming that 30% are interal?
Since you know the amount of traffic you want to present to each trunk, why do you need a formula to compute it for you. Voice traffic engineering is based on 3 facts…blocking, load, and # of servers (DS0)s and assumptions about the traffic model generally random arrival patterns, infinite amount of sources and blocked calls lost.
You do not have all three so we cannot compute this for you…but you do have the amount of traffic and the amount of traffic you want in each circuit.
3008×0.125 = 376 Erlangs
376/.8 = 470 circuits
Using the Erlang B formula we find that the blocking is incredibly low…You will have too many idle circuits and are in a sense wasting money.
The proper way to design it is by finding the amount of traffic and determining what your company wants as their Grade of Service(GOS)If we assume a P001 blocking rate, random arrival and 376 Erlangs of traffic you need approximately 425 DS0s.
60 DS0s @ P001 yields 40.794 Erlangs of traffic
I cannot answer yours because there is not enough information. How many days are in your month? 20? 28? 30? 31? Is the traffic evenly dispursed throughout the week or is it only dispursed through the business week? What percentage of the calls in one day are during the busy hour? most assume 17 – 20%. What is your grade of service? This will get us the lines and can be based on an erlang B formula…if want to take retrial rates into account you would need to provide the retrial rate and use an Extended Erlang B formula. To determine the amount of agents you will need to give us the amount of delay that a user can sustain, in order to find out the number of agents.