Restore to factory settings
Android
http://praxistipps.chip.de/android-handy-neu-aufsetzen-so-gehts_2792
iPhone
https://support.apple.com/en-us/HT201252
Windows Phone
http://www.windowsphone.com/en-us/how-to/wp8/settings-and-personalization/reset-my-phone
Blackberry
Because the Blackberry support websites are frequently revised, the links become quickly outdated. For this reason we have to ask that you research this topic on your own: Blackberry Support
You can streamline your search by knowing the exact model and operating system version. Help is available at: de.blackberry.com/support/identify.html