How can I avoid Premium SMS charges on du bill?

I am using iphone with du network. In my last months bill I have been charged AED 40.00 for SMS, the number mentiond in the bill is 2428. Upon enquiring du customer care told me that charges are for PREMIUM MT SUBSCRIPTION. But I do not remember sending any message to 2428 or subscribing for any sch services. How an I avoid these charges in future.