With the iPhone, you can store data like home page URLs, birth dates, instant message usernames and multiple phone numbers as contact information. The multiple phone numbers feature is especially useful because it allows you to store alternate numbers for a single contact. This way, you don't have to create multiple contact entries in your phone book.
Open the "Contacts" application. The "All Contacts" list loads to your screen.
Select an entry from the list. The contact's "Info" page opens.
Tap the "Edit" button on the toolbar at the top of the page. Edit options appear next to the contact's details.
Scroll to the phone number section. Tap the label of the second phone number field. The "Label" page opens displaying a list of label options.
Tap a label to select it. Some options include, "Home," "Mobile," "Main," "Work Fax" and "Other."
Tap the "Phone" field next to the label. Your on-screen keypad opens.
Enter the second phone number you want to add.
Tap the "Done" button on the toolbar at the top of the page when you are finished to save the number and exit the set-up.