I suppose it's because he knows you do care about him, and he misses that.
Whatever the reason, you know the answer. This guy is a train-wreck. Do you want to get back into the middle of it?
My second husband, who went back to drinking after almost dying of the disease, used to call me, and I listened and sympathized and encouraged for a while. Eventually it just stressed me out too much and I quit listening.
Look, there are a gazillion places he can get help if he wants it. He doesn't need a babysitter.
I would go with your instincts and quit interacting with him. If he sneaks a call through again, hang up, or if you want to be polite, say, "I cannot talk to you any more. I wish you the best, but do not call me again. I don't want to hear from you." And THEN hang up.