My ex and I have been on a joint account for our cell phone service for some time now. We broke up years ago but I didn't know I was able to separate my account from his. So I've just been sending him money for my phone bill ever since then. Several months ago (maybe even a year ago) he got fired from his job. He has not found a new job since, although he has been doing DoorDash. DoorDash wasn't enough to pay his rent (we were roommates for about a year until recently) so I had to cover his portion of rent multiple times. Usually he paid me back but not for the last month's rent. Well a couple months ago, my phone got shut off. Mind you, I've been taking my portion of the phone bill out of his rent. Turns out that he hasn't been paying the phone bill for months and we owe close to $600. I can't afford to pay even half of that bill with my current financial situation. So I just financed a new phone after a month of not having one. Now he is insisting that I take responsibility for the old phone and the balance that is on the account, even though his portion of rent was so much smaller than mine due to the fact that the cost of my phone was coming out of his rent. I can't afford to do this either. I offered to give him my old phone so that he can pay the balance on the account and cancel his line and just use the one on the phone that I have (the phone is not paid off so the line can't be cancelled once the balance is paid). So am I the asshole here?

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TLDR; I paid my ex for my phone bill but he stopped paying, we owe over $600, I just financed a new phone because I couldn't afford to pay what he owes. AITA?