Providers of local exchange services shall have access to local telephone numbering resources and assignments on equitable terms that include recognition of the scarcity of such resources and that are in accordance with adopted national assignment guidelines and commission rules. Additionally, all local exchange companies shall make the necessary modifications to allow portability of local numbers between different certificated providers of local exchange service as soon as reasonably possible after such portability has been shown to be technically and economically feasible and in the public interest.