Stop spam SMS alerts by only playing a notification sound and vibrating your phone for senders in your contacts. To make this app work as intended, disable sound and vibration in your normal Messaging app, and let this app do it only for senders in your contacts list. If you are using a version of Android earlier than 4.4, you may like to try the original https://f-droid.org/packages/ru.glesik.nostrangersms, of which this app is a fork.