Mom blocks father's phone number in child's phone?

Long story short- I'm seeing a man who has two children from a previous relationship. His ex got upset over something ridiculous and blocked both of our phone numbers in his son's phone. He's often home alone (or babysitting his younger brother, they are 13 and 8 years old) and is sick frequently, and neither one of us can call or text to check in on him. Is this legal? Obviously we'd like to be able to get in touch with him and its not right that she took it upon herself to put a roadblock in our communication (on a side note there are no restraining orders or anything of that nature that would warrant this action).

Is there anything we can do to legally force her to unblock our numbers? She is the custodial parent, though we see the kids 2 weekends a month and almost every day when they get home from school. Obviously a father has a right to contact his child. Any advice would be appreciated. Thanks!!