Usually the problem is the stability of the WIFI signal between your Smartphone and the internet router. Try optimizing your wireless connection.
If you have already optimized your WIFI connection and are still experiencing problems, try the following:
1. Make sure that enough bandwidth is available, both for up- and download. Excessive up- and downloading can have a negative effect on the call quality. A good test is to deactivate (not just minimize) all programs or devices that could be up- and/or downloading and to try and call again. If the call quality is okay now, you have found the cause.
2. If the call quality is lacking using the VoipTiger app for Android/iPad/iPhone, try the following:
a. Test the same account on a different Android phone or on a different iPad/iPhone.
b. For Android, download CSIPSMPLE APK from the Android Market, configure your SIP account in that application and then try to call using CSIPSMPLE APK.
3. Install the free softphone X-Lite on your PC and connect the PC to the internet using an Ethernet cable. Configure your SIP account in X-Lite and then try to call using X-Lite. A wired connection is always more stable than a wireless connection.
4. By default the codec is set to ILBC. You can change it to a codec that uses a higher bandwidth and less compression.