How to stop MTN beep from sending beeps?

I called a number when I had no airtime, and ever since then MTN beep has been attempting to send a beep to that number. I keep getting a message that reads "MTN BEEP: Your beep call could not be completed as the person you are trying to reach is not available. We'll try again soon. Thank you".

I keep getting that message every few minutes, even though I made the call about 24 hours ago. Help