Pearl Medallion affects only how much certain spells cost to cast. Spells exist only on the stack, not in players' hands (C.R. 111.1), so despite Pearl Medallion, you can't exile Teferi's Protection from your hand with Isochron Scepter, since that card's converted mana cost is 3 (C.R. 202.3), so not 2 or less.
The converted mana cost of teferi's protection depends on its mana cost, and teferi's protection's mana cost is always 2W. Pearl Medallion changes what you pay to cast this spell; it doesn't change the characteristic of the mana cost itself.
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