My standing policy for phones is that I expect people to manage their own availability, and respect that I do the same.
If I don't want to answer a call, I don't answer it the first time they call; if it's really an emergency, they'll leave a message or keep calling back. I only give out my Google Voice number, so I get transcriptions of voicemail, and I can silence groups of people, or only permit certain groups of people to call me at a given time.
If I want to talk to someone, I call them, and trust that they won't answer if they don't want to talk. It's absurd for me to be expected to work out whether it's a good time for a call or not, as I don't know their schedule, just like it would be absurd for me to expect anyone else to know my schedule and try to make decisions for me on whether I want to be called or not.
I've had several friends who have quite a bit of anxiety about making and receiving phone calls, religiously answering every call and stressing about whether or not it's acceptable to call others. I might be a bit idealistic, but this seems like the simplest solution to me, with the people who have the most information making local decisions for themselves.