By allowing a customer to retain his or her telephone number, regulators believe that it removes a major barrier to competition. The service, however, required telephone companies' networks to undergo reconfiguration to be able to process the network call path so that a customer changing telephone companies did not have to change telephone numbers. Some believed that customers would be less likely to go to a different service provider or new competitor if it meant they had to change telephone numbers.
It doesn't mean you can take your phone number anywhere. It just means you can take your number from company to company serving the same service territory.