Terrible service

I have proof that i paid my account. I reply on a mail from their collection department. They told me to phone MBD my account is handed over. I phoned MBD and i am not on their system no where. Collection department sent mail back they cant retrieve the proof of payment because the reference number is wrong? How can you hand me over if my reference number is wrong? On the 7th of December my account was one amount, today the 13th of December its another amount? How do they calculate this amount?