Say, you have already blocked a contact and want to unblock it on your iPhone 6. Well, To unblock a number / contact on iPhone 6 or unblock one of the numbers of a specific contact or block just one number of a specific contact, do as follows:
1. Open the Settings on your iPhone 6
2. Go to the Messages settings (Yes, Messages. It's easier to find than the Phone settings)
3. Scroll down to SMS/MMS settings and open the Blocked settings
4. There you should see the list of already blocked contacts/numbers on your iPhone
5. Now, to unblock a number / contact on iPhone, tap the Edit button on the top right and delete the numbers / email addresses to be unblocked.
Tip: If one of your contacts has more than one number, then to block just one number / email address of a specific contact, you need to delete the other numbers of that contact from the mentioned menu.