You can legally try to collect past due bills by calling the debtor.  But you cannot legally call them on the phone over and over again.  It is also against the law to threaten them with harm or contact them at work after they tell you not to.  In addition, the law says that if they write to you and ask that you not contact them at all, you must stop.  At that point, you can only contact them to let them know that you are suing them.  So it is a good idea to keep records of your calls and written communications to them.

If the debt is not a commercial debt, you are not supposed to contact their employer, except to verify their employment.

You should never send anything that is meant to look like a legal document when it is not.