OpenPhone is a standalone business phone line on your mobile device and it doesn't use any of the minutes on your phone plan.
It's completely independent from your phone plan.
In order to make or receive calls or messages, an Internet connection is required. You can use both WiFi and cellular data.
This way, you can use OpenPhone from your desktop or laptop as well!
If you'd like to forward your incoming OpenPhone calls to a different number, you can do so right within the app.
Since OpenPhone offers an unlimited plan, you don't have to worry about keeping a track of minutes or messages at all. Hope you enjoy OpenPhone!