Did you see this accident board said use SOS phone on M5!

[J500ANT]

On my way to/from the NE i've ben up and down the M5. Somewhere just south of the M42 junction is a sign (one on either side) asking did you spot an accident (I dont see many of these boards on Motorways) but what I was very surprised at was that it said to call a number or stop and use the SOS phone.

Now as I understand it you can't stop on the Motorway unless its an emergency, so I wonder what the legality of that board is?

Tony :)