If you can find one the LG Classic Flip would be my recommendation. It has the loudest volume of any of the flip phones I've tried and it's the only model that hasn't had any major complaints from my elderly customers. It doesn't have flip open to answer turned on by default due to the small outer screen that can't display the number of the caller but you can turn it on in Settings -> Call Settings -> Answer options.

If you can't find one at a reasonable price the TCL Flip 2 (-not- the My Flip 1/2) is the only other phone I would recommend for elderly customers. I can't speak to call quality as I haven't activated one for a customer yet, but it seems decently loud based on the startup and button sounds. TracFone's startup sound does sound a little tinny compared to the Classic Flip but it should be ok for calls. Both phones support speed dial.