Treat an IVR like agents, if you can queue calls waiting for an IVR port, otherwise treat like a trunk. Then use Erlang-C or Erlang-B calculators to determine the number of ports. Use the average port holding time of the IVR port and busy hour call volumes and IVR specific service standards for answering performance as inputs into the calculations. The Erlang calculations should give you the number of ports needed.
I read your previous reponse on how to calculate staffing requirements based on the predicted number of calls, duration etc. for each interval which the day is broken up in to. However if this is done as typical for every half an hour, does this take into account overflow calls that would arrive in one interval and would still be handled in the next interval? Is this an issue with using Erlang C or possibly even if Simulation were used by looking at half hour intervals by themselves? What can be done about this to make sure we don’t undersize resources?