i don't know if you know the app HeyTell?
well, i have to put internet on my cellphone to use that app.
but, i don't know if Heytell take internet everytime i open the app, or everytime i record a message.
So i don't know how many internet to take.
According to the apps' description and permissions, HeyTell requires full network access and requires your location, both approximate and precise to be enabled in order to function which consume data (or internet). The app has the permission to run continuously as shown in the diagram below. Click the image below to enlarge.
By the looks of it... The app will use data everytime you send/receive a voice message. Which is why it asks for permissions to access data as it pleases.
But it is like Whatsapp, in that it can seemingly operate purely on wi-fi, too. So as long as you are careful and make sure it's only using your wi-fi, you might not always need to use data. Try taking flex data so you can monitor and try to control your data use at first?