In iOS 8, you can make regular phone calls through Wi-Fi (if your carrier supports it). The feature is called Wi-Fi calling and is not very different from VoIP where you make calls through the Wi-Fi or data network.
Where it differs from VoIP (other than the core technical stuff) is that Wi-Fi calling doesn't involve any extra app, accounts or most importantly – expenditure. All it needs is a Wi-Fi connection, an iPhone running iOS 8 and a carrier that supports Wi-Fi calling.
Here's How to Enable Wi-Fi calling on your iPhone Running iOS 8:
Step #1. Open Settings
Step #2. Tap on Phone
Step #3. Under the label Calls, tap on Wi-Fi Calls
Step #4. Switch on Allow Wi-Fi Calls
Step #5. An alert will pop up. Tap on Ok. (more on this alert later)
That’s about it. Once you switch this on, you will be able to make calls through Wi-Fi instead of cellular. However, there’s a lot of subtext to this that you should know. Read on.
Currently, T-Mobile (US) and EE(UK) support Wi-Fi calling. Users who are on T-Mobile or EE can make Wi-Fi calls from their iPhones. You just make a call as you'd normally do – but the carrier name on the status bar will change to “T-Mobile Wi-Fi” (or EE WiFi). That's an indication that you are making a Wi-Fi call.
More carriers will be joining the elite club to offer Wi-Fi calling for iPhone users.
Interestingly, there could be device limitations too. For instance, in the case of T-Mobile, Wi-Fi calling is not available for iPhone 5 and I don’t know why that is so (not yet).
Alert: Wi-Fi Calling Will Disable A Part of Continuity
When you turn on Wi-Fi calling, you will not be able to use one aspect of Continuity: calls. Remember that feature where you can make/pick calls from any device? That won't work.
Again, I don't know if this is some sort of a limitation. Anyway, that alert in #5 informs of you this. And to enable Wi-Fi calling, you have to let go of the Continuity feature.
One scenario I can think of is that you turn on Wi-Fi calling only when you make calls. At home, when you are working on your iPad or computer, turn it off so you can get calls on any connected device.