Yes. According to the FCC: "Be aware that when terminating service with a wireless company, you may be obligated to pay any early termination fees under your existing contract. Also, when terminating service with any company, you are usually required to pay any outstanding balance owed. Review your bill or contract to determine what fees or charges apply. Once you request service from the new company, however, your old company may not refuse to port your number, even if you owe money for an outstanding balance or termination fee." and "Companies may not refuse to port a number because a consumer has not paid for porting."
From FCC web site »www.fcc.gov/guides/porta ··· roviders