Hola Amy

In the lesson we explain that if we are referring to an event still to happen you use the present subjunctive after cuando, and if we refer to an event that actually happens, then you use the present indicative, which is what we are doing in this question:

 

How would you say "They never answer the phone when I ring"?

Nunca contestan al teléfono cuando yo llamo. 

Here, we're saying that "whenever I ring they never answer" so it is something that "happens". (whenever something happens)

I hope this clarified it.

Saludos

Inma