It's called UMA... and your cell phone provider must have that service... if they don't you can not do that with your device... each provider has different ways they deal with billing of this service... T-Mobile USA has this service and if you pay for it then whenever you use UMA to make calls it does not count these as part of your normal minutes in your calling plan, if you don't pay for the service you get charged under your normal phone call plan.
Do you have to pay for the hotspot service, or just change your wifi type in the phone to "hotspot"? I've just been on my home wifi to make calls and it shows that I've been calling using my uma wifi.
One great feature of uma (besides not using plan minutes) is service in areas you wouldn't be able to normally as long as you have wifi.
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