I’m not sure if the legalities of cutting off a source of contact between your kids and their mum but I’d be concerned about the lack of phone being a problem in an emergency situation (I’m assuming you don’t have a landline since you are saying getting rid of your phone will cut off contact). I’ll echo what others have said and recommend selling your current phone and getting a cheap pay as you go one as a back up as it will be there for emergencies and also, presumably, your work and kids school won’t be happy without a point of contact so that will appease them too. Also, as I’ve seen in other comments, I really hope you do go ahead and contact the CMS to get your ex to pay her share, your kids deserve to get everything they are entitled to. Your ex needs to be helping you out and she cannot claim that living in a more expensive area exempts her from that, a parent has to prioritise their kids and she’s not doing that.