Nationwide Debt Collectors keep calling me

Nationwide Debt Collectors keep calling me after i have already scheduled a once off payment for what is apparently in arrears. I don't even know how my store card is in arrears in the first place when my payment plan is a debit order. So it is woolworths responsibility to debit my account each month. This is such unprofessional service and really annoying honestly. So not only am i frustrated with the fact that my account is somehow in arrears but i must be bothered with calls to settle my account when I've already arranged that!!!!!!!!!!!!!!!!!!!!!!!!