Hi Karl,
CUAC does not hold the call, the call is always controlled by CUCM and CUAC just sends requests to the CUCM to redirect the call.
It sounds like there maybe a CSS/Partition Issue here, by the sounds of things the CTI Port might not be able to contact the end point which is being transfered too. Ensure the CSS of the CTI Ports used by CUAC can contact the Partition of the phone.
Alternately, is it possible that the DN being transferred too exists in more than one partition and CUAC is transferring to the wrong one?
Either way I believe this is liekly to be a CUCM routing issue.