You could paste one below the other, and sort on phone number.
I'd expect you would want to examine the duplicates, so would include
a sequence  number from top to bottom (constants) and include that
as your secondary sort field.

Then see Chip Pearson's page:(s) on Duplicates
   http://www.cpearson.com/excel/duplicat.htm#HighlightingDuplicates
others can be fround in his
  http://www.cpearson.com/excel/topic.htm