Money chasers no service

I got a call from a sales lady. Offering me a phone and demanding i do a debit mandate.. Immediately after she called i got a better offer half the price same product. I have been trying on phone email and WhatsApp to reach them to say I cancel the application but no one answers or respond at all. I even left a message on their website. And I cant stop their mandate they put on my bank. People be aware they are only after money to take it from your bank.