Hi Carl, yes tried restarting, makes no difference. Does this phone have MMS function? I suspect long text messages converted to MMS and these are responsible for filling up the memory. Problem is they are not visible, so only SMS can be deleted. Only way to free up memory, at the mo, is to select "delete all" option.
I will speak with Doro technical support, hopefully they can explain and advise the solution. Cheers, Jon