The iPhone Contacts app provides several labels for numbers, such as main, mobile and work to identify different lines. What if you need to add a label not included in the list, to identify multiple mobile lines or a land line for a contact?
You can add a custom label to any number by tapping the Edit button while viewing the contact. Touch the label to the left of the phone number. Scroll down to the bottom of the list of labels, and tap Add Custom Label. Enter the custom label text and touch Save.
Your custom label is now added to the list of contact number labels, and can be selected to label any phone number.