Hi,
Thank you for the quick tip.

I believe a follow up is in need.

In case the manager asks for a new batch of random customers after a while,
The "on the fly" GUID(NewID), although it is generated uniquely per run,
might produce some of the customers that were on the previous run.

Therefore I’d suggest to add a BIT type column (0,1) to monitor which customers were called,
so you could mark them up and exclude them from your next runs.

SELECT TOP 5 PERCENT *
FROM   Data.SalesByCountry
WHERE Called = 0 -- 0 = not called, 1 = called
ORDER BY NEWID()

Make sure you update the [Called] column to '1'
for  the customers you called.

A select with output/update query will do it at one go.