I’m not from the US, but where I live it’s either (or a combination of):

1. Your contract runs for two years. You can cancel it before, but still have to pay for the first two years. Often prices depend on which category of phone you want (say 20€/month for the service, 25€ with a “smart” phone, 30€ with a “premium” phone, 35€ with a “power” phone,…)
2. You have two separate contracts, one for your phone, one for the mobile service. In this case you might pay for your phone 24 months, or 36, or whatever you agreed on and you can cancel the mobile service independently (assuming it’s not also locked to 2 years)
3. Some carriers even allow you to only get a phone without a contract for the mobile service.
4. If you finance a phone with your carrier, they’re legally bound to tell you what you pay for your phone monthly and how much for the service - there are many ways around that, unfortunately…

In any case, you get an unlocked phone.