If the "when to take the kids" issue is the only real one between you, well, consider that things could be worse, honestly. Take a breath and focus on that for a minute. Some older people, assuming she's older, really are not fond of their telephones and feel no obligation to pick up a ringing phone if they're busy, or even if they're not busy. They see phones as intrusive. My mom and her many friends felt like this, and I tend to think like that too, so it's not just the elders. I do agree that it's annoying when she wants to be in contact and then does not answer or call back, but just be aware that she may not even realize how much she doesn't respond to the phone.
It sounds like you and she could use a schedule. Rather than setting up her time with the kids casually "as you go," take a calendar, and work out the dates and times for her kid-time over two weeks or a month, then see her --- don't do it on the phone, see her, and tell her this is why you're coming over -- and say very cheerfully and positively, "I know you love the kids. And I really do appreciate your help. It's tough sometimes to work out the times, though. How about if we work together to set up your dates with the kids for the next few weeks and see how that goes, having it scheduled in advance?"
She may balk and say she wants to be more casual, feels like she's beiing made into a babysitter instead of a warm fuzzy Grandma, etc., but if you are very upbeat, don't force things and make her feel you really need her and the kids like to be with her, it could work. I would not get into some big confrontation about the phone thing, especially as you get along well otherwise; focus on the schedule idea. Oh, yes, your husband should help with this too! Ensure she knows: "We're not just treating you like a babysitter! We really want you to have the time you want with the kids, it's just hard these days with everyone's activities. We want to be sure you get YOUR time with them and this is a way to do it."
If she just refuses, give it some time and try again, reassuring her that this is for her benefit. I would not withhold the kids if she is balky about a schedule, though. You do like her, other than this scatterbrained and/or somewhat manipulative way she has with scheduling her time!