I agree with everyone ~ it's disruptive to both the little one and everyone else.

One thing I have learned through my experience of being a daycare provider with my own business ~ there is a difference between working with parents and letting them micro manage you and call the shots.
An idea if she seems to dig her heals in the ground after you tell her she can't call anymore, offer alturnatives like tell the Mom to make a special craft like a heart and she keeps one piece and the little one gets the other and when Mom picks up her little one the heart will be one again?? If this little one is suffering from separation anxioty then she is only making things worse.