I can't recommend a phone to you but try looking at the phones recommended by the RNID web site:
http://www.rnid.org.uk/shop/products/telephone s/
The cordless ones are on the second page.
Once you find one you like you can order online or try and find the same model on the high street i.e. Argos etc.
Has your mum been for a hearing test recently?
Would she not benefit from wearing a hearing aid(s)?
The Audiology / Hearing Aid department at my local hospital helped me enormously with getting digital hearing aids and any additional help I required.
I was referred to the hospital by my GP.
Good luck.