Still being billed years after contract expired

I have had the same cell phone number on contract for around 18 years already and originally took my contract out with Autopage. Around 4 years ago when it was time for my contract renewal I opted not to renew it again and paid all the amounts up until the dates they were due (I confirmed dates of cancellation, final amounts, everything). Everything was fine and I received no more accounts for a few omnths and I went onto prepaid. Then suddenly Vodacom bought out (or took over) Autopage (even though my actual number was a Vodacom number, my contract was through Autopage), and I started receiving phone calls from a collection agency saying that I still owed on the handset - which I couldn't have because I had completed my contract time in full and only then cancelled my contract (I eventually just paid them because I couldn't handle the fight anymore). After paying that amount, Vodacom then tried to claim there was still an amount outstanding on the phone account ....! I was speechless. Months after the contract was finished, all was paid and I had gone onto prepaid ... I have had countless arguments and additional charges added onto this account now because of my refusal to pay it. If all was clear when I was with Autopage then how come when they took over almost a year later all these amounts popped up? They have no explanation for me and I keep asking to speak to a supervisor and never get to do so. Also, this phone was originally in my ex-husband's name so they refuse to deal with me which is very irritating as it has been my phone number for 18 years. They can't deal with him if he is not here! I don't know what to do anymore.