She is your daughters Grandma. Put yourself in her position in 20 years time where it might be you wanting to see your own grandchild and feeling like you're not allowed. Are they within a reasonable distance travel wise from her?
I would suggest you make it clear to your ex MIL that she is welcome to see as much of your daughter as her son will arrange and that she needs to discuss this with him. Also make it clear that you are not prepared to go behind his back over this. From the sounds of it it is your ex who is not taking her to see her grandma enough. How much contact time does he get with your dd?
Make it clear to her any time that she calls that you have no objections to your dd seeing more of her, but that she needs to speak to her son to sort this out.
Then you need to tell your ex that his mother wants to see your dd more and that he needs to sort this out with her. Perhaps he can take her more often, after school once a week or something?
Is your MIL able to collect your dd from school once a week maybe and then your ex bring her back afterwards? What about your dd going to her dad a bit more often so he has more time in order to take her to her grandma's more often? All options for your dd to see more of her Grandma.
Show that you are willing to facilitate this, but that your ex has to do the legwork as far as visiting goes. If she keeps ringing you tell her calmly and clearly she needs to discuss it with her son and keep repeating that. If she won't listen then tell her you aren't going to discuss it further, to talk to her son, and if she doesn't stop then i'm afraid you have to put the phone down on her.
Perhaps suggest that if your Ex MIL wants to write, send postcards etc in between that she is more than welcome to do so. Grandma's love their grandchildren too. It's great really that she wants to see her, don't let your daughter miss out on this important relationship if there's things you can do to encourage it.
Best wishes
Gilly