Recycling of existing number

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