I you have a call that does not connect, or repeatedly drops, you or the other contact may not be on Wi-Fi or have a strong (three bars or higher) carrier connection. After attempting a call several times it may be helpful to close the app and clear the cache.
In Apple, double tap the Home button, find SC in page views and slide it up on the screen until it is removed. Then hit the Home button again to return to the Home screen. Now re-open the app and try again.
In Android, go the Settings, then Application Manager and select SC from the list of apps. On the details screen, click Clear Cache. Nor return to the Home screen and re-open the app.