I'm having a similar problem. To me it looks like the system can't find the CallID.
IVR Node: Unknown call
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns2:IVRInput xmlns:ns2="http://soap.com/pbx">
<CallID>22e77c4a4f79939d55c1898c7d1874de@10.1.1.212#5722c11005</CallID>
<Output>5630</Output>
<From>"J PORTER" <sip:8014071566@10.1.1.212></From>
<To><sip:5631@pbxnsip.nexsip.com></To>
</ns2:IVRInput><?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns2:IVROutput xmlns:ns2="http://soap.com/pbx">
<CallID>22e77c4a4f79939d55c1898c7d1874de@10.1.1.212#5722c11005</CallID>
<Destination>5630</Destination>
</ns2:IVROutput>
[6] 2007/12/28 14:27:07: Last message repeated 4 times
[7] 2007/12/28 14:27:07: UDP: Opening socket on port 61858
[7] 2007/12/28 14:27:07: UDP: Opening socket on port 61859
[7] 2007/12/28 14:27:09: UDP: Opening socket on port 61650
[7] 2007/12/28 14:27:09: UDP: Opening socket on port 61651
[7] 2007/12/28 14:27:09: Calling extension 5600
[6] 2007/12/28 14:27:09: Redirecting to external voicemail account 5600 destination sip:75600@pbxnsip.nexsip.com
[5] 2007/12/28 14:27:09: Dialplan: Match 75600@pbxnsip.nexsip.com to <sip:5600@10.1.1.51;user=phone> on trunk exchange
[5] 2007/12/28 14:27:09: Charge user 5600 for redirecting calls
[7] 2007/12/28 14:27:09: UDP: Opening socket on port 58796
[7] 2007/12/28 14:27:09: UDP: Opening socket on port 58797
[8] 2007/12/28 14:27:09: UDP: recvfrom receives ICMP message
[3] 2007/12/28 14:27:10: IVR Node: Unknown call