The carriers, in return, offer free or steeply discounted phones and accessories when you sign up.
Most providers have an early termination penalty.
This can range from a flat fee to a charge per month remaining on your contract. Also, if you break your contract, some carriers even make you give the phone back. It is a good idea to check into whether or not you can get a trial period of service to test out a plan. This gives you a limited window of opportunity to cancel service without any penalty if you find out that a provider just doesn't meet your needs. Otherwise, you'll have to wait until your contract is up to get a new deal from a different provider.