I would absolutely take the mobile phone and any other form of communication that he is attached to in order to get him to hold up his end of the bargain. You say that puts up barriers between the two of you, but those barriers are already there and he knows how to work you so that you don't take the phone. Give him the set of clear rules, clear consequences for him not following them and then you must follow through by taking the phone for the pre-determined amount of time. Yes he will be mad, yes there will be tension...but you are the parent and he has responsibilities that he must live up to. If your ex has him some of the time you must both be on board with the plan to present a united front.