Harassment from MBD in the workplace, although account is paid monthly

I started paying the account via debit order from December 2022 until mid April 2022 because my account was debited at random dates and not the date we had agreed upon. From end of April 2022 I started paying my account via EFT but still receive calls of non payment and reminders. Recently they have started calling me on my work landline which is a Call centre. I receive calls several times a week even though I make payments every month end. I have not consented nor provided them with my work landline because my employer does not approve of incoming personal calls. I have now been called into my managers office a number of times due to frequent calls from MBD which I feel constitute as harassment and breach of confidentiality as I am now experiencing prejudice in the work environment. The consultants confirm that my cellphone number is correct and on their system but choose to use a landline for reasons unknown to me. I have sent a number of emails to which there is no response or even acknowledgement of receipt... So hypothetically if I were to lose the job that allows me to pay them, how then do they suppose I would afford to continue to make payments to them? I have also requested a statement but have never received it.