If you are paying the full retail price for the phone, then you don't need to pay off the tab. But if you want the tab again, you would have your remaining tab balance added to the next bill. Being behind on your payment may limit your tab options.
You can get a new Tab if your bill is past due, depending on account history. But you will not be actually paying the Tab. You would be adding the Tab balance to the already past due bill. So you can't even pay the Tab without paying your bill first.