Thinking of some people I know who are making long-distance relationships work, it might be worthwhile for these two women to think about what modes of communication work well for them. For example, would it help them feel connected to send the occasional postcard, just "hi, I love you" or "this picture reminded me of when we met"? Not as the only means of communication, but an extra, along with phone calls or email or text messages or Skype.
Low-key answer-when-you-get-to-it IM/chat or texting, with the understanding that either person might walk away for ten minutes, or ten hours, can work for some relationships, but saying "I'll be right back" in any medium and not coming back is going to make the other person unhappy. (This means that I try to leave a "good night" or "it's my bedtime" even if neither of us has said anything in a couple of hours.) "I love you, I'm busy, talk to you later" can be understood/mutually defined as "I wish I had time to talk but don't know when I will" rather than "later today," but I suspect that with the history of glitches and resentment it wouldn't work for Ruth and Jessica.
Going back to the original question, I think it would be reasonable for Jessica to say something like "Saturday afternoon [or Sunday morning or the third Thursday of the month] is the time I'm most likely to be able to talk, can we pencil a phone date in for then and not make plans for other things without checking?" if there was a time that would work moderately often. But that is reasonable if she'll be available most weeks at that time; it's not reasonable to expect "save every Saturday afternoon for us to chat, I will be available about one time in five."