If the contact has more than one contact number, when you click on the entry in Contacts and then go on to click on the grey icon with the "i" , you will see a little grey circle icon with ">" alongside a displayed phone number. If you click on that it will offer you a selection screen to select the default number from a choice of all the numbers you have listed for that contact. If the contact only has one number this option will not reveal itself. It's not intuitive and you need to be told how to do it. 😀
