Hello Moose72: I'm sorry your dad is ailing. (He & I are of similar age, by the way.) My thinking is that calling is probably the best thing to do. The fact that your dad isn't tracking the conversation probably doesn't really matter. What's important is that he knows you care & you're keeping in touch as best you can. My thought would be that more frequent calls of shorter duration are probably the best under the circumstances. The content probably isn't important... it's just hearing your voice & having someone familiar to connect with that's important.

Of course, you can also send flowers or candy or whatever else you think he might enjoy. You might also consider speaking with the nursing staff. If there's one of them that has worked with your dad more than others, this person might have some suggestions as to what would be helpful. Unfortunately, I don't know if there really is a way to be detached & also be sustaining. They're kind-of at two ends of a continuum.

P.S. One very practical thing you & your sister might do, if you can, is to assemble a photo album for your dad. Perhaps your sister could take it with her if-&-when she visits him. I think this is often something that is done for people who are struggling with memory issues.