Both columns A & B have phone numbers. Each of the 50,000 column B does appear in the column A with 391,000 too.
I just want the 50,000 column B to have each number matched up with the same ROW as the 391k column A.
This way the same row for both column A and column B have the same value if the data is in both rows. If there is a phone number on the 391k list, but not on the 50k list, then in that row, column A would have a phone number, and column B would be blank.
Move Column B data to Column D
Then in Column B, use the following formula cell (B2) as an example.
=vlookup(A2,D:D,1,0)
That will match the value in A with the list in column D. Copy and paste special values and you are done.