If you use an Apple smartphone, the native FaceTime app supports voice-only calls over either Wi-Fi or your data stream, without using the cellular voice service. In this way, the calls are usually free. You do not need to add other apps.
This is what I have been using for three years now to call home from Spain. In my experience, it seems to works best if you are on someone else's free Wi-Fi, rather than using your phone's native voice capability.
However, I have had times when I THOUGHT I was making a free voice call over Wi-Fi, only to find out that my phone used my voice package. The discovery was the extra amount in my normally flat rate bill some weeks after returning home. Evidently the smartphone is in fact smart. It will find the best available signal and use it to place the call. If the Wi-Fi is weak, it will automatically use the voice circuitry.
Hope this helps.