Sort the data using telephone number as the sort field. Put a heading
like "check" in H1 and a variation of Ikabod's formula in H2:

=IF(G2=G1,"Duplicate","Unique")

which will report the second and subsequent duplicates. Copy this
formula down the column.

Then click Data | Filter | Autofilter to apply filters to each column,
and use the filter pull-down for column H to select "Duplicate".
Highlight the visible rows and Edit | Delete Row, then use the filter
pull-down to select "All". Your duplicate records will now have been
deleted, and you can remove the filters and delete column H.

Hope this helps.

Pete