So, you don't get all the features if you port in an existing number. How
There's really no point to this new option -- it's just a stripped down 'lite' version of the existing service aimed at drawing in people who were scared off by the "you'll have to change your phone number" nonsense. There's absolutely no reason to choose this option when you sign up. If all you want are the voicemail features, sign up for a "full" account (with a new number), then add your existing phone and switch on voicemail handling. That's it.
Actually, this new option is even worse: once you set it up, you can't go back to your carrier's voicemail system. You're stuck with Google's whether you like it or not.
And by the way, it's not full number porting, you still keep the same carrier. Not yet at least -- Google says that's on the horizon.
Today is a good day for information-gathering. Read someone else's mail file.