Again, please carefully read my post: http://www.obitalk.com/forum/index.php?topic=1051.msg86959#msg86959
Perhaps this will help your understanding:
All US telephone numbers are originally issued to Federally-regulated Local Exchange Carriers (LECs) in blocks of 10,000 (or sometimes 1000) numbers, by an industry organization in charge of the overall US telephone numbering plan (area codes and prefixes and their locations). The Local Number Portability (LNP) FCC regulations require that, in most cases, telephone carriers allow their customers to take their business elsewhere, to a different carrier, and in so doing, give control of their telephone number(s) to the "gaining" or new carrier. The original carrier always retains ownership of the number, but lets the other, gaining carriers re-route calls to their switches.
So: when you port a phone number from, say, AT&T Wireless to Google Voice's carrier, the two carriers work with the national organization that controls telephone call routing, to tell all of the telephone switches that, when a call comes in for that number, to re-route it to Google Voice's carrier. Once that routing change is made, the losing carrier (AT&T Wireless in this example) cancels service on that number at AT&T, since service is now being provided by Google Voice's carrier.
When you want to port in a number to Google Voice, and you submit a porting request, you are giving Google's carrier your permission to act as your agent, to contact the losing carrier, asking them to turn over control of that number to Google's carrier. After that happens, service is shut off on the losing carrier and activated on Google's carrier. The mobile phone on which you had the service will stop working as a telephone, unless you sign up for a new line of service with a mobile phone carrier (i.e. you get a new SIM and a new telephone number).
Google Voice requires that you first sign up for one of Google's free phone numbers. You can do your testing with that number before deciding if you want to port in your mobile number.