Below is a 13-letter phrase with no repeated letters and its space removed, and below it is placed the remaining 13 letters of the alphabet in alphabetic order:

PUBLICSERVANT
DFGHJKMOQWXYZ

It can be used to make a code so that vertically touching letters substitute, mutually, for one another, such as coding GROVEL as BQEWOH. The phrase PUBLICSERVANT was the key used in this encoding.

Below are the plain text and encoded text of three words, encoded using the same type of scheme, but using a different 13-letter phrase with no repeated letters rather than PUBLICSERVANT.

SYMBOL TEETHE WIZARD
VNPCHQ ETTEOT JXGUFK

What was the 13-letter phrase that formed the key to this encoding?

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From the 2019 Mensa 365 Brain Puzzlers for 2019 calendar, puzzle for March 5, by Fraser Simpson, Workman Publishing, New York, NY.