Traffic Engineering

Hi,

Currently, I have 3 new ITSP customers and they are planning to upgrade
their backbone network. At the moment, they have 3 E1 lines to Local
ISPs in Malaysia and planning to have 2 E1 connections to UUnet soon.
Anyway, is there any formulaor rules that we engineer to work on in
order to decide how much bandwidth required for VoIP business? For
example, if I am planning to have 1 million subscribers and 500,000
users will be active at the time, then how much bandwidth do I require
in order to offer good voice services? Anybody any idea? A lot of local
engineers keep on telling me that we can only guess for the bandwidth
and there is no exact answer? Is that true? I really doubt. Thanks for
your advice