Worse experience ever

I had a vodacom contract that was always debitted from my account everymonth my sim stopped working yesterday and i just been retrenched and i sent out my cvs with my number not realising its no longer working when i went instore to do a sim swap i realise this number belonged to someone else and they owe r6000 and claim i owe r2000 how is that possible and i called vodacom numerous times noting that i keep getting harrassed to pay this account by attorneys but it is not my fault now i cannot get my number back