A few simple actions, OP;
1) First, stop answering every time she calls. Answer the first time, then when you end the conversation, tell her you are really busy or about to begin something that will keep you from answering the phone, like work, driving somewhere, running errands or cleaning the house (running the vacuum, etc.). Be very clear that you won't be able to talk or even answer the phone for a vague amount of time.
2) Let her next 3 calls, or her calls for the next 3 hours, whichever is first) go to voicemail. Check them (when you have a minute) to make sure she's not freaking out that you're not answering.
3) Answer her 5th call (or the next one after 3 hours have passed). Respectfully humor her as usual, unless she is freaking out over the unanswered calls. If that is the case, gently remind her that you told her earlier that you were unable to use the phone due to whatever excuse you used earlier.
If she persists in her anxiety or anger over the unanswered calls, your grandma is going slightly insane. If she claims she doesn't remember you telling her you'd be unable to take phonecalls, your grandma has dementia. Either of these scenarios requires new plans of action for you and your family. Adjust accordingly and consult a healthcare professional. Dementia, anxiety and extreme emotions can all be signs of larger health issues.
4) If all is well on that 5th call, end the conversation as you normally would, then tell her (since it will likely be evening by this time) that you enjoyed hearing from her today, that you look forward to talking again tomorrow, but you are going to finish work:eat/go out/do chores/whatever, before turning in for the night.
5) Repeat step 3. Again, check the vms to make sure she's not having an anxiety attack. If she calls back more than once, see the last paragraph of step 4.
6) Assuming your grandmother has not proven herself to have dementia or another mental illness, repeat all over again the next day.
7) Visit your grandma more often. She's likely just bored and depressed, very common in old folks.