The decline button only performs one action, which is why it only has one icon.

It is visually consistent with the incoming call screen that every iPhone user knows. When you are on a call and another call comes in, you get the incoming call screen - not the existing call screen, which has completely different buttons like the number pad and speaker phone etc.

I think that’s where the confusion is coming from for most people - they don’t use iPhones and they don’t know what the “incoming call” screen and the “currently in a call” screens look like.

When you do, you see why this isn’t an issue.