Hi, here is my though:
0.05 Erlang per subscriber (just incoming)
25% of suscriber = out work place then a voice mail is left.
Traffic= 1500*0.05*0.25=18.75 Erlang
Ports Erlang(0.01, not delayed included)= 29 ports which makes sense to what Avaya propose.
I recommend to use this general formula and compared with commercial capacity. Memory for voice messages will be adress per the amount of messages you store per customer (again system capacity).
Hope it helps you o give more ideas.