Dealing with my ex calling during my custody time

She calls the kids like 5 times a week when they're with me. Last Sunday she called 3 times in one afternoon. I've tried talking to her about it calmly but she just says she misses them. I get it but this is my time and the kids are with me to spend time together, not on FaceTime. How do other guys handle this? Do I need to set a boundary in writing or am I being unreasonable?