My paid up letter

I have had the worst experience with Cellc, i paid my account in full and i keep calling the customer service department and sent through complaints via facebook their customer care email address and still no one seems to know what to do i have been sending my details back and fourth compromising my information for just a mere paid up letter, i have done everything im not sure if i should go via the ombudsman route for me to get assistance i have been calling since the 26th of July and no one seems to know what they should do, I just need a paid up letter

I guess even their customer review after the calls is disregarded as i have been complaining but still nothing