Does she have a phone designed for the hard of hearing? My aunt had one, it was actually easier to speak to her on the phone than shout at her in the flesh (refused to wear hearing aids).
No, because her hearing is perfect - there are just lots of problems with the phone lines

So, turned up this morning with my own mug for tea, dropped my coat on the doormat so I didn't bring it into the flat, just in case, washed my hands like mad, sat over 2 metres away for a chat and then told her I was going shopping. "oh, I'm coming with you" Major argument but I had no way of preventing her from actually leaving the house. So all I could do was ensure she walked well away from me and fortunately did convince her not to come into any shops (apart from the bookshop, where she spotted an elderly friend, also in poor health

) I was hopping mad - a morning booked off work (when I'm short of money anyway) and a 90 minute round trip, all to save her going out, and then she goes out anyway!
Her town was filled with elderly people, people in wheelchairs...all carrying on as normal. I give up.