Cordless landline telephone

Hi,
I am looking for a cordless landline telephone for my mother who is hard of hearing. However I want one that has a high call volume set already (not one that requires the user to press a “boost” button on the phone to activate the higher call volume).
The phone she has at the moment is 35 db but I would like to get one with 40 db or higher if possible. But the important point is that the call volume has to be set already, it must not be reliant upon any user intervention during the call (my mother won’t think of that).
Thank you.

This can be accommodated as we do:
Our adult son has a Family Plan account at AT&T. His mother, my wife has an Apple 11 Pro cell phone (I believe any iPhone since Apple 6 will handle it). She has Costco fitted Jabra Enhance PM receiver-in-Ear with Bluetooth connected system (similar to that provided by many HA manufacturers including my Costco fitted, Sonova manufactured, Kirkland 10.0T, hearing aids).
Leave the volume set as high as needed.