my husbands ex messaging me to communicate about the kids..

my husbands ex is messaging me and not my husband as a form of communication regarding the kids. I dont know why, it wasnt ever an arrangement and i just wish she would call his phone and not text mine. She doesnt have much custody of the kids at all (only every second weekend) shes had my husbands number the same number hes had for about 8 years yet continues to text my number. Her and I DO NOT get along. She repeatedly tried to get back with him and even turned up at our engagement party not invited! She has threatened me while pregnant and is a drug addict. The way its going it wont be long and she will lose all custody... which isnt a bad thing. but what can i say without stirring the pot to say thie is my number fuck off its not my business call their father....