Step.1: Open Settings on your iPhone
Step.2: Select "General"
Step.3: Scroll down to the end of General settings
Step.4: Select "Reset"
Step.5: Select "Erase All Content and Settings"
Confirm by pressing "Erase iPhone"*
*This option erases all data saved to the phone, including: contacts, music, apps, photos, etc. Make sure to sync your phone to your computer to create a backup prior to trying this method.
If you want to delete all of your iPhone contacts at once, you will can use a 3rd party tool like CopyTrans Contacts, "Cleanup: delete multiple contacts fast", "Delete Duplicates and Contacts - Easy Delete", or "ContactDel: delete all contacts with one touch".
The fastest and most convenient way to delete iPhone contacts - in my opinion - to use CopyTrans contact. This software allows you to select all contacts and click save your contacts to your PC before they removed from iPhone. This article describes three fast way to delete multiple contacts from your iPhone.
Way.2: How to delete all iPhone contacts with 3rd party tool?
Get the app from: Delete iPhone Contacts
Drag the "CopyTrans Contacts.exe" file from the zip folder to the desktop and start the app with a double-click
Connect your iPhone to the Computer
Select all contacts that you want to delete
Click the red "x" or use the right mouse-button
Confirm your choice and you're all set!
Way.3: How to Delete all contacts from your iPhone on iCloud ?
If you have synced your your iPhone contacts to iCloud or to your email account via Exchange, there is another easy way for you to remove all iPhone contacts at once.
1. On your iPhone, navigate to “Settings > Mail, Contacts, Calendars” and click on iCloud (or Exchange depending on where you sync your iPhone contacts)
2. Now make sure you move the slider next to “Contacts” from ON to OFF. Upon prompt, tap on the “Delete from my iPhone” button
This will delete all contacts on your iPhone which were currently synced to iCloud or your email account.
Way.4: Delete All Contacts from a Jailbroken iPhone
Jailbroken iPhones with Cydia installed have an additional option to delete all contacts at once. The "Erase Contacts" app from Cydia will get the job done. Follow the steps below to use the app from Cydia:
Open Cydia and search for "Erase Contacts"
Install "Erase Contacts"
Locate the "Erase Contacts" icon from your home screen (Contacts icon with a red 'x' through it)
Select the "Erase Contacts" icon*
*Launching the application will immediately erase all contacts from your phone without giving notice or asking you to confirm. Be sure to have a backup of your phone before downloading and trying this app.
Apple Official website: www.apple.com
Tips: If you accident you important contacts from iPhone,there are two ways for you to recover iPhone contacts: “Recover iPhone contacts directly without backup”; “Restore lost iPhone contacts from iTunes backup files” to get back all of you deleted contacts.