Hi Charizard
I can't give you any false reassurance here, because I don't know why they are ringing you.
Sometimes the DWP send out these letters because something has been flagged on the system as a potential fraud issue, but when they investigate there is nothing amiss and I know of a couple of people have found out they weren't claiming benefits they were entitled to.
Try not to worry. Just have any information to hand about your financial situation.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems