As of Android 12 and up, and all iOS versions, unfortunately you cannot set Fongo apps as the default calling app.
However...
- Some devices allow you to long-press on a number you want to call, then choose your Fongo app to place that call
- When using some voice assistants, you can specify that your assistant uses the Fongo app to make your desired call
- Example: "Call Jim McNeil using my Fongo Mobile app"
Single-tapping a number, and/or saying "Call ________________ " to your voice assistant will always use your device's native phone app.