Owe a little money, don't pay late fees, but won't be required.

Will drag you into the blacklist

In the future, if you apply for Unicom card, you must pay off the previous arrears and late fees.

If you owe more (a colleague of mine owed more than 800 a few years ago and didn't pay it back. A few months later, he received a subpoena from the court, asking you to pay the arrears together with the late payment fee.

If your card is registered in real-name registration system, of course, don't worry.