What, exactly, is the violation here?

If you want to produce copies of some work, you need permission from the copyright holder. In the case of GPL'd work, the GPL represents that permission.

Whether Skype were violating copyright law depends on whether or not they were actually copying anything. If they were merely selling phones they bought from the manufacturer, then copyright doesn't come into it and they don't need permission from the copyright holder. In that case, they're *not* bound by the GPL, although the manufacturer is.

On the other hand, if Skype were customising the manufacturers phones with their own GPL-derived software, then they would need permission for that, and they *would* be bound by the GPL.

Seems to me that the first scenario is the case here, so I'm not sure what Skype did wrong.