I just got some Vtech phones and the ringer is pretty loud, but it is also a lower pitch ringer which helps. It has a pretty large face on it so you can see when someone calls too (lights up when it rings). It is cordless but it is pretty good. It also has a good handset volume control. I can hear pretty well on it and I have about 70% hearing loss. I got them at Office Max about 6 months ago.
The set I have is this one:
http://www.vtechphones.com/vtechpho...?event=ehCatalog.productDetail&ProductID=1464
I have also had very good luck with Uniden phones but could not find one with enough handsets that had the DECT technology (where you can use it with a cell phone) but I am guessing your Grandma won't care about that.