Unknown contact list, under label

I am using iphone 6 and in the contact list when u go to any individual contact u have various options under which u can save a number like home , work , mobile and so on. Its here I have a issue when I go to select these options under Label apart from few default labels and add custom label I have a contact list of atleast 500 people. irony is these contact list is not mine. I had deleted this list twice deleting it one by one since there is no option of multiple select, but once I am thru these process again after some these list come back again.

what can I do to clear it off once and for all? Kindly assist