FaceTime will not connect with one contact

This is my Mom's iPad. FaceTime works fine as she can call other people; however, every time my Mom tries to FaceTime with her Mother, the call won't connect. My Grandma is able to make FaceTime calls with her other kids / contacts but when she tries to FaceTime my Mom or vice versa, we can hear the call come through; however, it won't connect. I've tried googling this issue, etc. to find a fix but nothing is working. My Grandma and Mom are both set up as Email contacts as neither one has a cell phone / cell number. I have already deleted the app and reinstalled it a few times, powered down the device, etc. Nothing works. Has anyone else experienced the same issue and resolved the issue ? If so, can you please let me know how you did it or what worked ?

Thanks,

Karen in Canada