Per National Mah Jongg League rules, if you call a tile but have not exposed yet, you can still change your mind about calling it. However, once you've exposed, you're committed to an exposure, though you can change the number of tiles exposed.
The way this rule manifests in our game is that when you press the Call button, you can choose pung/kong/quint or you can cancel (which is the same as if you had clicked Ignore).
Once you choose an exposure, you can change your mind on the number of tiles exposed (for example from kong to pung) by clicking the Change button above the exposure, but the Cancel button is not available since you've committed to an exposure.