the only 2 carriers that currently offer wifi calling are Sprint and T-Mobile. As long as you turn it on and can connect to wifi, simply make or receive a call from any platform or carrier normally. If you get a good wifi signal you will have amazing voice quality on the call. A weak or slow wifi signal then not so great. This is great for many indoor situations where you may not get a strong cell signal.
When you turn wifi calling on for the first time you get a new PRL installed on your phone. That's it.
Here is an FAQ
These slides show the activation of wifi calling for the first time
O2 offer it in England.
T0Mobile only mentions iPhone (no specific model) and iOS 8 so t would seem so.