The first thought that comes to mind is to make sure you are up on all the current patches, particularly in the PBX. Secondly, using the idea that the simplest answer is always best, it has been my experience that this kind of problem usually goes back to the agents sets, specifically, the headset. In centers where there were multiple types of headsets, I have seen problems with the programming of the headset on the agents set. This can cause the issue you describe. Also, if the sets are starting to get old, I have found that the headset cord is the first thing to go bad. As it does, this causes the set to go into a not ready condition.