It's easier to send SMS to above number. Send "Help" to above number to get instructions.
Formatt: CMP[space][your landline phone number][space][complaint code]
CMP 051123456789 BB
BB means broadband
They may SMS you back to get more information like send 1,2,3 about different options for complaint etc. Then finally, they'll SMS you with complaint number when your complaint is registered.