Add new number to existing iPhone contact, replace the old number

Today a friend called me from their new cell phone number. After the call, I added the phone number to their existing contact. Afterwards, I could not determine which of the two phone numbers listed was the current number that they had called me from.

If I navigate to "Recents" and click on the call, it lists both phone numbers and gives me no indication which was used for that specific call.

What is the easiest way to determine which phone number was last used with a specific contact?

When someone calls me from a new phone number, what is the easiest way to overwrite their old phone number with the new one?

I'm using iOS 8.3 on an iPhone 5

The blue number (the bottom one in your screen capture) is the one which made the call. The same works for emails in contacts that have more than one.

Here for example, I have a contact with a landline (home) and a mobile phone. Incoming and outgoing calls to the different numbers of the contact are highlighted in blue.

This is a screenshot of a call made to the mobile number of the contact (not an iPhone, its my grandma):

This is a screenshot of a call made to the landline:

Also, if you missed the call, the number calling will turn red instead of blue.

This is a screenshot of a missed call from the landline: