Poor service from MTN SOUTH AFRICA

I was using an MTN number but my phone got lost and i could not do a sim swap as MTN was not giving me a hard time to do a sim swap. I ended up buying another sim card and using my old MTN number for watzup purposes. I am still using the old MTN number but to my suprise MTN has decided give my my number to someone else while i'm also using it without consultation. I phoned MTN call centre and i was just that there is nothing they can do, i must just change my watzup number. My question is that are they suppose to very before giving the number away. I was just told that the number has been recycled. I feel this is also a violation of my privacy as now that person will be receiving my msgs i will receive her msgs.