I'm having the same problem on a brand new system with plenty of CPU and memory. I have two voice modems (Modem 1 - Hayes; Modem 2 - Broadxent). Line 1 will answer but when I move my phone line to modem 2, I get [not answered] when using both the simulator and when calling from my cell phone. What's odd is that the caller ID registers my cell phone number but then IVM doesn't answer the call.
I'm in the beginning stages of setting up a pretty complex system with a large number of OGMs so I'd like to get this sorted out before I get too far into this.