My mother gets a busy signal every time she tries calling me?

For the past few days my mother has received a busy tone every time she had tried to call me, we can still text and FaceTime. We have Both checked our phones to make sure we didn’t have one another blocked. We both have the latest updating system. As far as I know this is not happening to any of my other contacts. I have tried deleting her contact and re adding it after turning my phone off for awhile and then back on. But it still didn’t help. I also tried turning it to airplane mode for a few minutes and then back to normal to see if that would help. Any suggestions? I have tried everything I can find online to try and fix this problem. Our carriers also says it’s not on their end.