Pick you battles.....They are getting older and soon will have cell phones and will walk away on their own. You can text them soon too.
You could file in court to have mother not interfere with phone calls (but will it actually stop her)?You can ask to have counseling and have calls when kids go to the office.
Send them letters with the phone calls so they get mail, another form of communication.
Do you have contact with the kids school/teachers? Many schools/teachers have websites and apps they talk to parents on (Remind app is one of them used here). Ask the teachers if you can be added so you have info to talk to kids about.
Keep trying and just ignore ex trying to butt in. Talk about school, activities, upcoming next trips....
You can try sending ex a
certified letter stating "ex, I have been trying to communicate with the kids but you have been taking over the conversations when I am trying to speak to them. If you continue to interfere with the communications with the kids, I will be forced to seek court intervention. The kids need both parents in their life and I try my best to keep communication open even with the long distances between us. " Something like that....but pick your battles and only you know if you should spend time and money on this.
Good luck!