Can I legally withhold my child’s cellphone in my home (consequence for grades) if the other parent pays the bill?

Your best answer to this question is to allow your son the use of his cell phone at specific times so that he can speak with his Dad AND get Dad on board with the idea of limiting son's cell phone use as a consequence for bad grades and violating the school's rules re cell phone use on campus. While Dad bought the phone and pays the bill, he should be receptive to limiting the use of the phone when that is appropriate because that means the bill should be smaller. Even though you have sole custody, it is in the best interest of the child for both parents to be supportive of consequences for inappropriate behavior as well as applauding good behavior (like good grades). You are both still parents and your son needs to know that he cannot play one parent off against the other.