The FCC requires phone companies to allow you to move your phone number to a new company to encourage competition. If you have control of your cellular service provider account, you can port your number to a new provider. You can shop for the best deal and move your number to a new provider at any time.
Home phone numbers are portable, too. You can port your number from the traditional phone company in your area to the cable company or to a cellular provider.
Note that your phone number and your cellular provider are not the same as your cloudy account for your smart phone. See Smartphones: more than just a phone number.