Cancellation of contracts

Called the callcentre on to enquire about my contacts and send and email on 2018/01/19 to cancel contacts with them as from the 2018/03/01. When enquire I get told that they do not accept written cancelations and that I had to contact the call centre to verbally give notice of cancelation. This is unacceptable as i have given them more than 30 days notice as required. The small print of virginmobile only states that I have to give 20 working days notice and so does the consumer law. Nowhere it states that I have to do it verbally or on exactly the 30 days before contracts expires. Virginmobile now wants to charge me an extra months payment. Poor service delivery.......................