If the customer puts the leader on the phone. How should I answer, because this is a small question, so I don't need to bother the leader.

You can tell the customer: I have the right to solve this matter, and I am sure I can give you a satisfactory answer.

Then, do your best to help him.

If the customer is still stubborn, then ... small things will become big things, you can't hide them, you have done your best to bear them. At this time, it is logical to invite the leaders to come out again.