A friend has caring responsibilities for her mother, who lives on her own a few miles away. They keep in touch by landline telephone, but because of her mother's hearing problems it is very difficult sometimes to pass important messages. What is needed is a way for my friend to type a short message (for example "10.30" or "Tuesday") which will appear on a large display next to her mother's phone while they are talking - the reverse is not necessary. The care agencies and Social Services have no knowledge of any such device. I wonder whether a Pi-based solution might be possible.
My first thought was a pair of Pi's communicating via USB modems, but because the text link is needed at the same time as the phone conversation the normal protocols won't work. In theory I suppose a low-bandwidth link using a low-frequency FSK signal might be workable but would need to be engineered from scratch; ditto a 'chirp' protocol using short-duration tones.
Another possibility might be SMS based. If it were possible to link a cheap phone on a PAYG contract to a Pi-based display unit at the receiving end, my friend could send a text.
Has anyone seen anything like this, or have any thoughts?
BTW there is no internet available at my friend's mother's house.