I think I ran into something similar working on Mobility.
On the phone device properties page, the "Built in Bridge" options is disabled by default (at least on CUCM6 and later), and this may be the mechanism that the softphone is trying to use to perform this operation.
If that doesn't help, it might be that the call he is on is on a different line appearance than his ICD extension being controlled by the Cisco Agent Desktop (if that is the method you are using to de-queue calls).
Just a suggestion.
Kevin