Hello and welcome. You have come to the right place. I just found this board a few weeks ago and it's been wonderful to make new online friends who know all about what you've been through and are all about helping each other.
You are dfinitely, definitely doing the right thing not accepting his calls. You hit the nail on the head -- you are not responsible for his poor choices.
I'm sure if you've read some of the other stories you know about detaching. It means that you still love your son but you're going to face that you can't fix his problems for him. Doing that just continues the cycle of you enabling him and him abusing your love. Instead you will let him face the consequences of his choices and you will not allow his bad choices to continue to deny you your life. It is much easier said than done, we know that here and are ready to listen and cheer for you.
Many members have found Al-Anon meetings helpful.
Welcome and good luck.