I have $80 left to pay off on my tab. I would like to get a phone for $0, with a $300 tab. I also want to use my name plan and number. Will the 300 tab go ontop of my remaining $80 tab, or will I have to pay off my first tab before getting a new phone?
So as long as your current phone plan is 30$ or more you can keep your plan and move up to tab medium which has the 300$ subsidy. Since you have 80$ left you that would just mean you'd be eligible to use 220$ towards a new phone and would have to pay the 80$ out of pocket. This assumes the phone you wish to buy costs 300$.
You always keep your number as long as you keep your service.