Plain Old Telephone System. Provides your real Dial Tone. Sadly, most payphones give you a fake Dial Tone now, as a red box prevention measure. You can hear the phone dial the number for you.

POTS is also an acronym for the expression 'Plain off the Shelf'.

This expression is rarely said aloud, as it is generally used in email or message forums.

example: "The twelve hour battery life is great. Plus, when they run out you use POTS batteries to replace them!"

Many companies are saving on R&D costs by building new things (like the new space shuttle) with POTS parts.