Blackberry BIS charges on an Iphone contract error

Upgraded from a Blackberry to Iphone and guess what - MTN is charging me for the BIS service that I am no longer using. This is the second time it has happened on different contracts. I wonder whether this is a standard practice. Unless a client queries the costs they will continue to pay for BIS even though you no longer have the blackberry. Also no surprise the call centre is still not answering since my last complaint . 45 minutes later and they have still not answered or they keep dropping my calls.