Friday Fun ~ THe Magic of 8

Multiply your phone number by 8 (disregard the area code.)
Write down the following three numbers:
          Your phone number
          The number 8
          The product of your phone number and 8.
Add all the digits in those three numbers.
If the sum is more than one digit, add again.
Continue in this way until a single digit is reached.

What's the digit?
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ANSWER: 8
From Mental Magic by Martin Gardner, Troll Books