There is good reason for that. You know the little pipe that juts up through your roof? That is to ventilate the plumbing, so nothing siphons or gets held from a vacaum. Well, that pipe is usually metal and could conduct lightning into your house. It's unlikely, but if it happens, it would be better for all living things to be as far as possible from the water lines.
A similar logic for not being on the phone, lightning could come through the phone or electric lines. Since many phones now are cordless, that seems like less of an issue than it once was.