If the phone you have is a GSM phone, you can buy another phone on eBay or through a dealer and then just switch the SIM cards. You will keep your existing plan and have a new phone. If you are due for a replacement phone from your carrier, chances are you would have to sign a new 1 or 2 year contract. So, best is to buy a phone from a third party. If it is a CDMA phone, it may be more difficult, but I believe they will just transport your phone number to the new phone without renewing the contract.