There are two modes for deleting:
1. Normal Mode : Normal mode shows available contacts in your phonebook and deleting contacts will remove them from phonebook, but it may (not necessary) be possible to retrieve them after delete.
2. Advance Mode : Advance mode shows all deleted contacts from your phonebook, which you may not see but still available in phone memory. Advance mode will try to remove those deleted contacts from your phonebook to cleanup your phonebook and contacts deleted in advance mode wouldn't be possible to retrieve. Its recommended to keep your mobile internet on while deleting contacts in advance mode (as in some cases it required to have mobile internet to delete contacts permanently).
If you are changing your phone and want to give your phone to another person, but before giving your phone, you want to remove / delete all your contacts from phonebook, then this app is just for this purpose, just one click and all contacts are removed / deleted from the phonebook. Please make sure you really want to delete / remove all your contacts, otherwise it won't be possible to get back your contacts once delete, unless you have backup available.
As it deletes / removes contacts all at once, so it deletes contacts very quickly.
Share with your experience of this application, you may also send us your suggestion to some new interesting features, we will surely try to add them in coming versions of Delete All Contacts app.