I bought a phone on tab medium on March 17th. If I go into a bestbuy today, will I be allowed to pay off my tab and get another one? Will I have to call in first to pay off my tab? Is there a charge if I call in to pay off my tab?
You can upgrade your phone as often as you like! If you pay the phone in full at best buy today, then you don't even need to pay off your tab. The tab is really just the amount of money you can take off of the phone when purchasing it. Only time it really needs to be settled is when you want to cancel your account.
That being said, if you do wish to upgrade and use your tab with the new phone then when you are purchasing the new phone, they will only be able to put the difference of your tab's maximum value and what it currently is at. For example, you have a tab small that is at -100$, then you can only take off 50$ of the cost of the new phone to put on the tab.