It sounds like it might be Nicor Energy. A guy came to my home around a week ago. Said that he needed to see my bill and blah, blah, blah. He kept trying to tell me that they were part of Nicor. I told him to leave. I get a call around once a week
Rang 3 times and hung up. I wasn't going to answer it anyway. In this day and age, do yourself a favor and only answer calls from numbers you recognize. If they are legit calls they can leave a message. To do otherwise will open you up to hundreds more calls because you are a potential uninformed victim.
The person that stopped at your home was not NICOR. Their representatives will always be in a NICOR car and produce a NICOR ID. Further just for everyone's safety, never give anyone your personal info online or over the phone.