I am happy to let you know that I finally win this job. I beat down IBM, Periphonic, Voicetek, Intervoice.
For your situation, you first use Erlang B to calculate the IVR port. That is 1,200 mutiply 40 seconds then divided by 3,600 seconds. Then you have to assmue what % of these IVR ports will transfer to your agents. Then you can use Erlang C to calculate how many agents are required. You need to input in Erlang C the average waiting time for an agnet to answer.
You can also work in the opposite manner by the 35 agents. Then you answer to call volume may not be 1,200 calls per hours. Hope it can help. If no. I can work it out for you.