If you use the nextel's WLNP Eligibility Check tool, you can put in your phone number, then click "Check Eligibility." On the next page, check the box(es) next to you number(s), and click "Continue." You will then see a name like, "Global Crossing" or "XO California", "Bellsouth Mobil GA" (for my cingular 415 number), "Pacific Bell WRLN" (for my parents 415 landline).
Compare this to: Fone Finder which still has "Pacific Bell" as the Telephone Company, even though I ported this number to SunRocket over 2 years ago.