I am so sorry you are living in this hell. You did the right thing to call the authorities when he threatened suicide. You should do that each time. You must not let him move back in. When he calls if he can't be polite you can say "I cannot talk with you when you are shouting or threatening. We will talk when you can speak politely. Do not call back for at least 24 hours (or two hours or a week or however long you feel comfortable). I will not answer before then. Let him know that if he threatens suicide you will call the police because you love him, and because suicide is illegal.
Other mothers don't let their sons come home under these circumstances. You can see that that has not worked for you or to make him a functional adult so far. Something has to change, and right now it is the communication between the two of you. You have a right to insist that he speak to you politely. You are entitled to some peace, and can tell him you will answer in 24 hours and speak to him if he can be polite. Stick to it. Hang up and turn off your phone if he starts shouting.
What an awful awful crisis. We can all sympathize. My son never threatens suicide, he just makes gestures...the worst being when he slashed his throat. Nowhere near a real attempt, but really awful to see.
We are all with you. We will give you our thoughts and reactions, and the best advice we can muster. Please keep posting.
Echo