Unfortunately I don't know a way of forcing an instant answer. I know at one point I discovered a key in the registry for the number of rings, but I believe there must have been a validation check because it seemed to just reset back to two on it's own and I didn't notice any difference.
I believe the limitation is in place since Caller ID is not passed on until the pause between the first and second ring (on a traditional analog system), so IVM is forced to delay the answer in order to get the Caller ID. But I agree with you that in many cases this is unacceptable and there should be an option to disable it, especially since VoIP calls send the Caller ID at virtually the same time the call is initiated so the ring delay is completely unnecessary.
I once managed to get IVM to answer instantly while playing around with the time conditions, although I can't remember how and don't know if it would still work in newer versions.