Errors might appear in the agent desktop for call control issues.
Unavailable in current state.
Error making call.
Operation failed. Try phone controls for this action. If the problem persists, sign out and sign in again after your calls. (holding / retrieving / conferencing / transferring / ending call)
This means that the agent tried to take an action that is not allowed until he is in a certain state - for example, an agent cannot place a call from the READY state.
The destination may be busy or the number may be wrong.
There is a race condition. You already tried to perform an action on the call but the event did not come through.
Avoid actions which are not allowed in a given state.
You may need to disconnect the call and retry.
Verify if your intended operation has completed. If yes, the call appearance on the UI should be updated accordingly. If not, then try hard phone controls for the intended action. If the UI is not updated accordingly, then sign out and sign back in after you have completed your call.