Any time a FG is missed when the spot of the FG was within the 20, the other team gets the ball on the 20.
From NFL Rule 11, Section 4, Article:
Article 2 Missed Field Goals. If there is a missed field-goal attempt, and the ball has not been touched by the receivers
beyond the line in the field of play, the following shall apply:
(a) If the spot of the kick was inside the receivers’ 20-yard line, it is the receivers’ ball at the 20-yard line or
(b) If the spot of the kick was from the receivers’ 20-yard line or beyond the receivers’ 20-yard line, it is the
receivers’ ball at the spot of the kick.