My favorite single line telephone is the Western Electric/AT&T/ITT/Cortelco 2500. The new ones made by Cortelco come standard with amplified handsets, as well. These phones are super durable which sounds like it might be important for your situation. Just look for model numbers starting with 2500. A good one is 250044-VBA-20M.

If you have any questions about the 2500 set, there are people here that know those phones inside and out!

-Nelson