Debt collection on settled account 7/8 years old

Being harassed for an alleged outstanding vodacom accont that was settled and closed in 2019. They are unwilling to give me the information I need to follow up on this. Vodacom says there is no outstanding fees or costs. This place is ridiculous as I get get upwards of 5 calls a day regarding this. They request a REF number which vodacom cant provide me with as i am mo longer a client, then they are unwilling to help me get the supposed REF number i need to get this sorted. How can they come after me after 7/8 years for an account that was closed and settled and I have received NOTHING from vodacom in those years. Smells like a **** but I need the incessant calling to stop.