NO ANSWER - NEED URGENT CANCELATION

URGENT. I contacted Netstar to cancel a debit order they are taking. Big was my suprice when they told me they cannot as I need to cancel it with Telesure who I have never dealt with nor heard of before. I have tried many ways, times and duration contacting them. They do not answer their phones or I get a response "The number you have dialed is not available, try again later". How do I get my cancellation through. I am not working anymore and need my cancellation urgently.