Forget the fact that the transport media is IP, understand the behavior of the VOIP as..
TDM —VOIP —TDM connection, the same calculation applied to TDM circuits applied to IP circuits once the only thing that changes is the transport media and the compression provided by the codecs on the media gateways.
follow the result of the 0.35 milierlangs that you used to have on TDM, however this will work only for Prepaid calling cards, Corporative users or Home users, if you try to apply the calculation to wholesale carriers you will find that every carrier will use the available ports in a 1 to 1 relation begining PEAK-TIME.
Regards,
Saul Bejarano