Allow calls through in silent mode after multiple calls in Lollipop?

I'm not on the Moto X anymore, but my sister is. She claims that on KitKat, she had it set so that if someone called twice in succession while in silent mode, her phone would ring for the second call. Now on Lollipop she can't find that option. Is this no longer possible, and if it is, how can she do this?