There are a number of ways to adjust sound volumes on your iPhone. The volume buttons on the left side of your device can control "Ringer" volume (phone calls and notifications) as well as "Media" volume. Ringer and Media volumes can be set at different levels at the same time (eg. one can be muted while the other is loud).
To control the Ringer volume of the Fongo app you must make sure you're adjusting the Ringer volume control setting. This setting can be adjusted from your iPhone home screen or within the native iPhone phone app. The onscreen display will show the word "ringer" as you change the setting (see picture below).
Recommendation: set your preferred Ringer volume and then disable the button Ringer volume control setting to ensure you do not accidentally turn down your phone call and notification volume. To do so, go to the iPhone "Settings" screen and select "Sounds". Under "Ringer and Alerts", slide the "Change with Buttons" tab to OFF.