On Android, the ringtone for incoming Sure Send calls was barely audible, no matter how high the ringer volume was set. The app was playing it as if a call were already in progress, so it came out of the earpiece at call volume instead of the speaker, and the ringer volume had no effect on it.
The ringtone now plays like a regular phone ringtone: through the speaker, at your ringer volume, and it stays silent when your phone is on silent or vibrate. Once you answer, the call works as before.
This needs the latest version of the Android app.