Yes, they support multiple phones. Check the Amazon link that portman provided. It appears that all the major cordless manufacturers have sets that support this now. It's been pleasant to use since it (I'm using a Uniden set) has "just worked" for me since day one.

I'm actually going to set this up for my elderly mother for Christmas who has a regular landline but also a "dumb" cellphone. Her hearing is not that great so if the cellphone is not in her immediate proximity, she'll miss the call. In this case, I'm just interested in the redundancy for phone service in case of emergencies. It doesn't change how she will use the cordless phone at all but does provide us(her children) with more piece-of-mind. It really is this good.