Depends on how you have it setup. If you are setup on a wireless network then the app uses the wireless network to make/take calls. If you enable the option for 3G/4G it will burn your data package as you are off a wireless network.
If VOIP is turned off it will use voice minutes. If VOIP is turned on it will use your data plan (if you are not connected to wi-fi). So turn VOIP on if you are on a decent wi-fi connection and you won't use either voice or data minutes.
To help clarify a VoIP call is "voice over internet" so packets of data travel over the internet. This could be either your wifi, or 3g/4g network. The latest version of the Softphone will recognize which signal is the strongest and is capable of switching from one to the other. So for example if you are at home/office or any location where you have a strong wifi signal, it won't be using your 3g/4g data plan, but if you travel outside of that area, and have your phones 3g/4g (data) turned on, then it will use your phones data plan. As mentioned above there are a variety of configurations which allow you to control if/when your phones data is used. Let us know if you still have questions.