Account arragement

I sent an e-mail and requested an arrangement be made on my account, i then received an sms stating the amount to pay which is R1000.00. I assumed that my arrangement was made and continued paying the R1000.00. Im now being charged telephone and sms as well as other charges. This is not fair as Ive never received a letter of demand or phone call. I want to pay off this account but they are making it difficult for me to do so