I am in a situation where I need to install a Call Button system for my elderly mother. I was going to just wire buttons to M1 inputs but the reporter intercoms have some very thought provoking features:
Battery operation so I can put them in critical places like the shower as well as out on the porch or even in the garden.
Voice communication allows for a description of the help required (instead of panicing to run see what's wrong)
Battery also ensures operation during power failures.
My ecommerce store IntercomsOnline.com sells a lot of the wireless intercoms to the caregiver market. The WireFree Outdoor Intercom, which is designed for front door use, is also being used inside next to beds since it has one large button on it, it is very loud, and is nice and portable. Just stick some rubber feet on it and it won't slip around. Like you said, it can also be used in the shower. There's a picture of this application here: