You will be able to communicate with a good audio quality using a WiFi connection or with a 3G/4G network as long as it has a good signal. If you lose the WiFi signal during the course of the call, it will be automatically reconnected using the mobile one after 4-5 seconds.
In order to use the app over your data plan, you need to check if your network operator and your rate allows you to access to the VoIP traffic.
The minimum data connection requirements for a correct use of the app's different functionalities are the following ones:
In 2G networks, you could notice little problems during the course of the audio-calls. You could not establish video-calls with this kind of network.