It's also possible that the woman in the song is a former lover or partner of the man who calls on the phone....but if so, why is she now living in such a confined way, as would seem to suggest an elderly person who is alone? Why the rocking chair? Why wouldn't she just go out and find someone else?

I think the "things that go unsaid" are that he has other stuff in his life that matters a lot more to him and that takes all his attention now, and he can't really be bothered with more than a token call now and then, in which case he has really not much to say to her anyway. Those things can't be said. They hurt too much.