Unicom sent a notice that my mobile phone number was in arrears, but I never used Unicom's number. What should we do?

First of all, you should make sure whether this is a letter sent to you by a liar. Unicom usually has a professional person to call you to say that you owe money, instead of sending a letter. If it is really Unicom, then you have to think about whether someone has used your ID card to run a Unicom card. If not, then call Unicom customer service and let them know!