I think some people find it a comfort to think they're being hospitable even when they're not at home. I had to stay at my mother's to take care of her after she had surgery. My Aunt was out of town for several weeks visiting her granddaughter. When my mom was feeling better and not needing full time care I'd go to my Aunt's and watch her TiVo just to get a break.

While I was there, my Aunt would call her house phone and tell me how her trip was going etc. It didn't even occur to me that I shouldn't answer the phone until I read this thread. She fully expected me to, which is why she called her house phone rather than my cell phone, but pondering it now it seems rather presumptuous. If someone else called I simply took a message and gave it to her when she called me.