How can I change Contacts.app custom labels automatically using AppleScript?
Here is an example (quick scribble) of how to get and process the labels.

tell application "Contacts"
    set theNumber to "079"

    set (label of phones of people whose value starts with theNumber) to "test"
    save


end tell


Set theNumber to your own number.

AND I WOULD BACKUP THE ADDRESSBOOK BEFORE YOU START PLAYING WITH IT.

Update.

I have change the script to chaco if the phone number starts with rather than contains.



'label' = an items label. i.e 'mobile','iphone','home page' 

Phones = the phone numbers of a person/contact's entry

people = plural for more than one person/contact.

value = the value of an item in a person/contact entry - in this case the phone number

In the set command the code between the brackets ( ) is run first. 

Meaning the reference to  phone number's label in whole of the address book that starts with theNumber is gathered in a list.

Then the surrounding code tells each reference to change to test.