The default phone number is used when you initiate a call or send a text message using the Context menu that opens when you tap and hold the contact’s entry.
Open the Contacts application.
Tap the name of a contact in the list to view its details.
Tap and hold the phone number to use as the contact’s default phone number.
Tap Set as default number in the dialog box that opens.
The default phone number is indicated by a checkmark.