hotspot means using say a laptop to connect to the phone to use its internet. not supported out of the box. against the terms of service. but there are ways to do it.
hotspot does not mean wifi calling. vm does not employ wifi calling, nor do any of the major carriers (femtocells excluded..)
there are apps that allow you to make / receive calls over wifi. in terms of reliability, your mileage may vary.. grooveip, nettalk, skype, hell even google talk has voice capability now.