Assignment of existing number to someone else

MTN has given my existing number that I've been using for 20 years to someone else and they are causing havoc in my life. My life has been linked to that number since 2002. I learned yesterday that when people call me my number takes them to someone else who is not me and this is not a cross line. I say this because I started receiving calls and messages to pay debts that don't belong to me but to this said person. How does an existing number get assigned to someone else while I'm still using it? This is an 073 number that I've had since 2002 (TWENTY YEARS) but ported it to Telkom I miss very important calls and my privacy has been greatly compromised.