Mr price mobile harrassing my mother weekly over a bill that was settled in 2015

MR PRICE mobile agent called my mom in 2015 and sold a cellphone over the phone and it was delivered within few days. My mother made an EFT payment for the full amount but two year later they called and said payment was not made, they made two debits from her account, one was reversed but the MBCDL lawyers are calling her every week (for three years now) demanding that she settles the bill with intrest. We sent all the proof of payment and bank staments but every week a different pearson calls.