Horrible distorted song instead of ringtone.

Then people call me they reach a horrible distorted song, with the suggestion that they can download it by pressing 1. Most people put the phone down thinking they reached the wrong number. I did not ask for it, and I do not know how it got there. I was told by the MTN call centre to send an SMS to 4100. I hope it disappears and that I won't be charged for the SMS. If MTN does charge me, then this amounts to ******ion in my view.