As far as I know, when you port your number to GV (or any "phone provider") then you're not able to use that number on the previous service.
For example: you have (123) 456-7890 on Verizon, switch to sprint and port the number over. You now have the number on Sprint...but if you wanted Verizon service again you'd have to get a new number.
The same basic principle is in effect with GV. When you move (123) 456-7890 to GV, it is then GV's number, not VM's. So if you wanted to continue to use VM's services...you'd need another number.
Believe this is correct. If you port your number then you can't use it on the previous service. Sprint is the exception since they have some integration with Google Voice.