Particularly in the past - the standard way to buy a phone was to sign a two year contract -- and to then get that new phone for free, or $99, or $199. This is an incredibly enticing deal - particularly considering that the mobile carrier's true cost to buy you that phone might be $300 - $500 more!
You get a huge discount up front, and the carrier makes back the cost (and more) over the course of your contract. To keep you from trying to duck out, hefty early termination fees apply.
This subsidized purchase price is all fine and dandy - if you take advantage of upgrading your phone as soon as your contract is up, or even hounding your carrier for early upgrades. (Store managers can often authorize this up to 6 months early!)
But if you still have a phone you bought on a two year contract more than two years later, you are giving a huge gift every month to your mobile carrier. You are essentially continuing to make payments, even though your phone is already long paid off.
If your phone is over two years old and you are out of contract - STOP overpaying!
If you like your carrier, consider getting a new subsidized phone cheap and signing a new contract if they still offer contracts (many carriers have gotten away from the practice). You might very well be able to sell your old phone for more than this upgrade costs you.
Or - keep your phone, and switch over to a cheaper plan with special rates for out-of-contract phones. Many carriers now offer cheaper monthly 'line access fees' for devices that are off contract.
Also be sure to check your carrier's current rates - many of them have switched up their offerings, and their more modern plans may actually reduce your rates substantially. Even if you are in contract, the carrier may be able to switch you over.
Your carrier isn't going to do this for you automatically, but a phone call can save you substantially.
If you want a phone upgrade and do not want a contract, most carriers now offer attractive financing plans too called 'Equipment Installment Plans'. These separate out the financing of the device from your monthly service plan.
But whatever you do - don't keep paying more each month for a phone you bought two or three or even four years ago!