The iPhone makes it simple to add new contacts directly from recently used phone numbers. The process in iOS 7 has been revised, but is still relatively straightforward. To add a new contact from the Phone app:
1. Navigate to Phone -> Recents.
2. Scroll down to the phone number you wish to add to contacts.
3. Tap the blue information circle (i) to the right of the new contact number.
4. Scroll down to Create New Contact.
5. Enter the first and last name or other relevant data.
5. Select Done in the upper right corner to add the contact.