The £500 is being demanded to unblock his line.
I suppose, technically, as he's leaving for Plusnet, he's invoked the early termination fees. The figure seems spot on, £570. ( £29 x 18 ) + £50 = £572.
He may have been
told the figure was because he ran up a large bill last November, but the amount is too close to his early termination fee amount to be a coincidence.
So if he wants to get reactivated with Talktalk he needs to clear his old debts first. If he lets his move to Plusnet continue, so be it, but the £570 debt still stands.
Oliver.