weird message 'delete ALL stored messages from your phone!'

My girlfriend had the same today. (Pic below)

The phone showed 7.5GB of free space.

This lead me to believe that the message was not an Apple message (also just based on the look and poor format of the message)

She mentioned that this has happened once before.

We noted that the "Dismiss" button did not actually do anything and the we had to shut down and restart it.

She is also with the same carrier.

This leads me to believe that messages are from the carrier.

Maybe the messages on the server are not being deleted or something of that nature.

I might post on other local 'famous washing machine brand' forums and see if they have any ideas.

What jailbreak are you currently running on your iPhone?

That looks like what is called a "flash message". Generally, they are only sent by the carrier. However, there are ways to send them as pranks. It especially seems to happen in the U.K. Someone is trying to scare you.