Is it legal for the father of my children to change his phone number so I have no way to contact him?
Is there currently an order in place that requires him to provide you with a current number? If so, then he is violating the order if he changes his number and does not provide you with the new number. He could theoretically be held in contempt of court if he continues to violate the order.
Co-parenting means he should keep you up to date as to his phone number. There is nothing "illegal" about changing your phone number.