it is a very picky process... but it will work.
even when you configure it right the first time, it wont seem to engage the first time when available.
here is what i have done to get it working...
configure the setup and save... make sure it is set to "whenever available" as you have done.
after saving, soft reset.
turn on your wi-fi
go into voip config again and just save (without any changes)
go to your phone dialer...
now, instead of sprint / or verizon... you will see the name of the VOIP provider you gave (assuming it is able to connect to that provider)
when dialing, hold your phone backwards... sound comes out the rear speaker and is very quiet.
After switching to Samsung, I find myself asking... WHY did I stay with HTC for so long?
... NEVER going back!