I get harring calls from my ex father-in-law, is there a way to stop him from doing this? (Australia)?

I am separated from my wife for 8 months, we have 3 boys in our marriage all under 5, she has the youngest 1 I have the older 2. She has been getting her father to harr me over the phone in an attempt to give the kids up to her or him in which her father has allegations as a child abuser & she's been in trouble by the Department of Child Safety from her child currently in Fostercare & my youngest son. I've currently stopped this by changing my phone number from both of them but if I get custody of all the boys, I think I am required to give my number out to her but she will most likely give her father my new number & it will start all over again. Any suggestions?