I am moderately deaf in both ears and have middle and HF loss to both. I also suffer tinnitus.
I am a great fan of loop systems. I went to my eldest son's school Christmas concert in the local church. They had a loop system which was fantastic. Switching both aids to the T setting meant whoever was talking into a mic felt like they were sitting right next to me talking loudly!
In fact, my aids picked up a bare whisper from one young girl to another standing near the mic.
She said to her friend, "I need a wee!!"
If you can get a phone with a coil built in, they are ideal, but you have to set the aid to the T setting, which can be fiddly, esp. if your Ma suffers arthritis.
The other option is a cordless phone with speakerphone capability. I have a Panasonic one which works well on the speakerphone setting.
I am a member of RNID and get copies of their mag "One In Seven".
They do regular product tests & have in the past done a test on HA compatible phones.
Unfortunately, I have binned all my old issues, but if you get in touch with RNID & ask them for a copy of their latest HA-compatible phone test report, I'm sure they will be able to help.
They may even be on-line.
Another good idea is to talk to them for help & advice about other products that make life easier for the HOH. There are advisors who make home visits, but I'm not sure who they are engaged by, whether charity or local authority or NHS.
Eddy, I take my HA out of my left ear when speaking on a mobile: believe it or not, I can hear better without it. It's by no means perfect, but much better than with it in place!
EDIT: see these:
http://www.rnid.org.uk/shop/products/telephones/