Unfair customer treatment

I have a personal loan with this company that's been running for 2 years successfully where they debit the repayments from my Bank account until recently when they decided not to debit and started sending me messages claiming overdue amount. I took the liberty of contacting them to investigate. I was advised that the debit order was returned unpaid. This was a shock of my life, ever since I opened my Bank account in 2004 I've never had any debit orders bouncing. When I requested the consultant to investigate because even on my Bank statement there is no debit order that was returned. Without any explanation, the consultant hanged up the call.

I waited expecting to be contacted back but no, they decided not to give me answers nor at least phone me back. What now, must I phone again and waste my own money??

This is so disappointing to get such treatment from a Company that claims to practice best customer service.

I expect not to see any interest added on my loan account due to this error that nobody can give me answers to.