Is the son considered a Dayyūth if his divorced mother speaks with men over the phone?

If the mother speaks with non-mahram men without necessity, she should be gently and respectfully forbidden from doing so, bearing in mind the special consideration due when dealing with parents. If she does not stop, then inform relatives who are capable of preventing her. Your denunciation of this matter absolves you from the characteristic of being a cuckold (dayyooth).