Just my advice
#1 send the phone back to the aunt and let her know that you do apprectiate her help but you can no longer afford even half of the bill. I'm sorry, but the rest (cancelation fees) is between your ex and her nephew, expecially considering that you did not want your son to have it in the first place. If she asks, a simple"please take this up with him because I did not make the arrangements for the phone to begin with". the end
#2 no contact means no contact. Remind yourself that his business is no longer your business. Every second you take to wonder what he is up to is wasted time. Catch yourself, stop, bring the focus back to you, your son...heck whip out the budget and start adding up the saved money from that 1/2 phone bill...figure out how many months it would take to save for a minor splurge for you and your son (nothing super expensive, just set a goal,maybe give him a set cash number and see if he has ideas). Use that wasted time on him towards something positive and attainable.
#3 breathe, just breathe. Life is complicated and sometimes gut wrenchingly painful. Mourn, grieve. Do your self and your son a huge favor and find atleast one silly thing to do a day (standout in the rain, stomp in a mud puddle, slide down a hallway in socks, sing offkey loudly). The beauty of life is that as complicated and crazy as it can be, you can count on the uncomplicated silly things to be there for you to enjoy when you are ready. What would grandma say in all her aged wisdom? What would you say to your best friend if she was going through this now? Be your best friend. Be kind and patient and nonjudgemental with yourself, whether you believe it or not. ...you just need to keep moving forward even in the rain.
bottomline: what is perceived as overwhelming is usually a good indicator that you need to reach out for support...guess what, you are reaching out and here we are in all our messed up complicated lives doing our best to give you that support. Take what you need,leave all the rest...You are stronger than you even realize yet.