Whenever you receive or make a phone call on your iPhone, the device stores the associated number in your call log. Phone numbers that are linked to your existing contacts are also retained. However, you can make the iPhone “forget” any phone number not associated with a contact by erasing it from the call log. For a phone number connected to a contact, you can remove it from the device’s memory by accessing the contact’s information and manually deleting it. Writer Bio

How to Make the iPhone Forget a Phone Number  9 Steps  - 88