As you meet new people and enter their contact information into your iPhone, the occasion may rise that you need to add a special prefix to a phone number to ensure successful dialing. Whether the number is international or simply long-distance based on your current area code, adding prefixes to phone numbers is as simple as editing the contact.
Touch the "Contacts" icon on your iPhone home screen.
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Scroll through your contacts until you reach the contact that requires a new prefix. Touch the contact name to open the entry.
Tap "Edit." Add the prefix to the phone number by touching the number and tapping the appropriate keys. Remember to add a "+" sign if the number is international.
Tap "Done" to save your changes.