Sure! Two options- If you are in a plan with someone else in a contract, and their up for an upgrade, you can take their upgrade, and it will just be the same number, and a new 2 year contract (Depending on your carrier). You can just go to your carriers store and get a phone using someone else in your plan's upgrade. (Non carrier stores i.e. Best Buy, Apple Store only accept your own upgrades- and not early upgrades by taking someone else in your plan's contract). Ordering online works too. (Call customer service)
Another option- Pay Full Price! Although this may be very expensive, this also works if you are not up for an upgrade and can't use another plan.
Hope this helped you! Who is your carrier? Thanks.
Yes you can, it has to be on the same frequency as the provider of the plan that you have now. It can be expensive, but you can get real deals on phones on eBay craigs list kijiji etc rather than buying one from the dealer.