If I am reading correctly -
https://www.fcc.gov/guides/portability-keeping-your-phone-number-when-changing-service-providers, the first paragraph says:
Under the Federal Communications Commission's "local number portability" rules, you can switch telephone service providers for wireline, wireless or Voice over Internet Protocol and keep your existing phone number if you remain in the same geographic area.
IOW- if the provider you are choosing already has the exchange you need, they must be able to port your number. Now some simply may not offer your local exchange- I'm not sure you could port MUrray Hill 5-9975 to Vonage :) *.
*Extra credit for anyone who recognizes the number :D