When busy with a task, especially in Sales Oriented projects, you want to have everything in one screen that relates to the specific customer. In this case I would use the example of an Outbound Call list (each call is a task).
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List item Add multiple tabs to a client with additional client info (like recent purchases, associated contacts, prospect value)
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List item When needed to expand the Client info when busy on a task, show only in Modal Window with the details, so, you can go back just by closing the modal. Preferably (especially in this time of advanced UI development) leaving this up to the user, so making the size of the contact window more customizable, and what info to show in it.