I recommend a system that has a base station and the ability to register and ring to a number of handset around the house. Most of the 2.4ghz and higher units have both good sound and fairly good range. I use mine almost 400 feet out in the orchard, but it's pretty weak way out there, still useable though.
In addition to the units listed by others, take a look at the
several Panasonic models, that's what we have, that take up to 6-8 handsets. It does have answering included, almost all of them do, but we don't use that feature, or many of the others. You just get the features along with the key ability to have multiple handsets.