Hard of hearing customer - how can I get help from OVO Customer Support? Named contact not able to make changes for me

My elderly mother is severely hard of hearing and cannot cope with telephone calls now. 

She officially named/agreed with OVO for  my brother to be her representative to deal with her account however he was told on a later call that he cannot discuss/change her direct debit as she asked him to do and that instead my mum would need to telephone(!) to get a number for collections (or whatever the payment dept calls themselves now) to do it herself. The other option offered was for my brother to become financially liable and be named on our mother’s bill. 
 

All my mum wants is to do is reduce the new direct debit amount as she is substantially in credit. 

It hasn’t helped that my mum has been telephoned by OVO since and had messages that she cannot understand and this whole experience has her sick with stress. What part of “cannot hear” does OVO not comprehend? Why can’t her representative actually represent her?

We didn’t have this representative issue with a different power provider, my sister in law was able to fully manage her elderly parent’s gas/electricity on their behalf without having to be liable for the bill, so it is possible. 
 

I cannot find any information about whether we can access OVO colllections/payments via a text relay service of some kind but it would be far easier if my brother could just be allowed to deal with it all for her, as previously arranged. 
 

Any suggestions?
 

(As an aside, I’m aware that customer services must be inundated at the moment and emotions are running high after the past few and continued stressful years but lack of available, consistent access for hard of hearing customers has been a problem across all utilities for years. I for one am on the OVO priority service because of my poor hearing and like my mother have previously had voice messages left and been telephoned to check if our electricity is back on etc, calls that we cannot properly comprehend can be both upsetting and worrying.)