Call quality on the mobile app is highly depended on the network, very similar to using the desktop app. The quality of your phone's data signal can make a big difference. There are a lot of websites and apps that you can use to test your phones signal.
I personally use an app called speedtest.net on the Google play store but you'll find many choices.
If you have wifi available, it's a good idea to switch to that network as it will often provide better upload and download speeds.
Here's an example of two tests that I did. The first one is from a wifi signal, the 2nd from my cell phone's data plan.
You can also go into the mobile app's call settings and control if/when the device should use Wi-Fi or the RingOut option. (3rd image below)