A good rule of thumb: genuine official business is always conducted in writing. Phone calls and visits don't create a paper trail. They can't be saved in a file. So people who are owed and are trying to collect money for a legitimate reason or trying to sell a legitimate product or make contact about a legitimate legal matter send letters.

Obviously, parties like your doctor or plumber or personal lawyer or employer will call you because they're calling about matters that can be handled quickly and don't implicate legal rights or obligations. But if someone you've never spoken with before calls you or knocks on your door, unless they're in uniform with ID, tell them nothing except to send you a letter.