2 months asking for paid up letter

Im so irritated and fedup with MBD first of all they took over on my cell c account they said I owe cell c when contacted cell c they didn't see that missed payment and they told me that they closed my acc since I paid it off and ddnt want to renew contract with them. My credit score was on average very good and paying well. MBD took over and messed up with my credit score fine I paid the money because of their irritating phone calls and threads. It was around June if I'm not mistaken the last payment. From SEP I'm asking them to send me the proof of payment or paid up letter everytime I call either my calls are dropped or they will tell me that they will write an email to supervisor according to my knowledge of emails it doesn't even take 3min sending an email. Why does it take soooooo long now to send me the letter? I believe it now when people are saying MBD are ********. The last call I told them that I really can't be begging for the letter while I was "owing" they were on my neck like a rash.... I'm very angry they have poooooooor service