This happened in our primary school. The child never did own up but we had several reliable witnesses and the child's behaviour was a give-away. The phone was found and the parents called and brough in. The child remained in school and that was the end of the matter. Not sure if I would have dealt with it in a 'stronger' way (was not in headship at the time) as the staff member was very upset. If I were you, I would feel very uncomfortable teaching that child. Can you have a different group?