Quell des Schmerzes

An effect that causes you to pay life uses the word “pay.” If an effect allows you to choose to lose life or to be dealt damage without using the word “pay,” you’re not paying life.

The first ability of Font of Agonies is a triggered ability, not an activated ability. It doesn’t allow you to pay life whenever you want; rather, you need some other way of paying life, such as with Blood Crypt’s ability.