I came over the following problem (in IPCC 4):
Sometimes a call that is queued will simply be disconnected from the system. As far as i could find out, this happens on Select Resource Step, and it only happens occasionally (when there is a large number of calls in the system). The call is simply abandoned, however it was not the caller that disconnected the call while waiting.
The agents to not have any Cfwd configured on phones and Partition and CSS settings seem to be ok. These are the only two causes that i know of, which could cause such behavior.
Could this be somehow connected to the fact that there are 200 CTI ports configured in system, but there are only 150 IVR licenses?