How's wi-fi calling?

How is the wi-fi calling on this phone?

My roommate is in love with the webcam and is deciding between this and the HD 7 windows phone

or the g2

thing is
our coverage sucks in our dorm....i have at & t and i have spotty coverage

and with tmobile it is non-existent coverage
but i hear with wi-fi calling it elimates dead spots

1)Are there any distortions in the phone calls or are they clear?
2)are there any fees as well to use this service?

 

so it works even if you literally have no coverage...as long as u have wi-i?

 

wi-fi*