The app uses the Internet connection to request the calls to Infocaller servers. However, once the calls are requested, no Internet connection is used. Thus, you will always hear the person you’re calling with the best digital sound quality.
The amount of data used to request calls is very low so you should not worry about data quotas.
If you want to call from abroad with Infocaller App and you don’t have any Internet connection (WiFi/Data), for the moment you won’t be able to make calls.