Dunns not treating an old customer well

My mother has been using Dunns account for decades and she has been paying regularly, this year she was told that she didn't pay the account on May of which we did because it's us as her children who are paying, we were told to send proof of payment and we've been sending those since then, we would be told that they'll get back to us after 24 hours and nothing has been done, the account is still in arrears and my mother is still getting sms's thar she must pay that money, I just called their call center number and the consultant is still saying they'll get back to us in 24 hours and when I ask to speak to the relevant person she says she can't do that I'll just have to wait there's nothing she can do, I mean how many 24 hours passed since May this is frustrating especially for my mom who's elderly and a pensioner, obviously someone is not doing his/her job here and now it's affecting the whole family