{{{Nora}}}
erika, surgery-ma for your mom.
Teppy, I agree that it sounds like your dad is lonely, which makes things harder. Could you warn him ahead of time, by saying something like "I'm going to be really busy for the next few days, so I won't have time to talk again until Thursday," and then just don't answer the phone form him until Thursday?
I don't really have anything useful to suggest because, while my mom has boundary issues, she doesn't expect to talk to me more than once a weekish, which is fine with me.