BF3 is an exception to the octet rule, so it only has 3 bonds with no lone pairs. Because of this the Boron atom is electron deficient, but not so much that it is really unstable. The boron can accept electron pairs so that it then has four substitutents bound. It's ability to accept electron pairs makes it a Lewis acid.
PH3 on the other hand has a lone pair of electrons and fulfills the octet rule. It already has 8 electrons, so it cannot accept another pair and is therefore not a Lewis acid.
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