When your phone is on a WiFi network it should not use your mobile data, unless you have configured it to use mobile data when a WiFi signal is poor or slow.
If you are concerned about using up your mobile data I recommend leaving it on for 10 days and then comparing that data used by the Vonage app to other apps on the device. It all depends where you go and what you do in these areas. If you don’t take any calls or mobile video meetings then you will use very little.
Take my phone as an example. For the current billing period the Vonage app has used 16.9 MB of mobile data, Facetime has used 59MB, and my podcast app (Pocket Casts) has used 7.4GB. I found that info here (https://support.apple.com/en-us/HT201299 ). The Vonage appss data usage included any calls and the data used to stay connected to the service. To be honest, I am almost always in at a WiFi location. My daily 25 minute commute through poor cellular areas is about the only time I use mobile data.
To understand how much data you really need read this article by Tech Radar. It will compare various types of app usage and the estimated data usage, TechRadar: How Much Data Do I Need.
If you are still concerned then I recommend you read up on how to manage your mobile data via Low Data Mode. The “What to expect in Low Data Mode” was really helpful.
You can also choose not to install the Vonage app on your personal mobile device.
Another option is to disable the app from using ANY mobile data.
On Apples iPhone: Settings > Cellular > then scroll to the app and click the toggle
https://support.apple.com/en-us/HT201299 See section “View how much data you're using”
On Android read about it here: https://support.google.com/fi/answer/9458407?hl=en