For some reason IVM prevents you from answering before 2 rings in general.
For analogue systems, I can understand this since the Caller ID doesn't usually get pass until just before the second ring.. but for SIP there is no need for this restriction and in most cases is unwanted. A while back I think I got it to answer more quickly by playing around with the date/time restrictions and/or some tweaks to some registry keys (can't remember how I did it).