Unclear on the principles

We offer a VOIP service that is hardware based. We sent our subscribers a little router that connects through ethernet to their highspeed modem, and lets them connect their computers to it. it has a phone jack to which you can plug a normal phone, which will then be used to operate the VOIP service.

The other day a customer called in, after installing the service himself, to ask why, when he puts his cable modem on stand-by his voip phone would stop working.

I find this baffling. He has already made a causal relationship between his turning off his internet service and the voip stopping, and he knows, since he plugged it in himself, that the voip requires internet... How could he not conclude himself that he shouldn't turn off his modem?

I guess it is too much to ask that people behave or think in a logical manner.

I know this story isn't quite funny ha-ha. I'll do better in the future, I'm sure.