A work item is a transaction you are actively working. Inbound calls, call backs and SMS are all work items. If you receive a voice item or sms item, it will automatically put you in "Working" Status, you will have to complete this item before you will be able to receive another call. 

Be sure you are closing your workitems as soon as they are finished. Especially SMS. Once you reply or send an sms, close the workitem and disposition it immediately.