Tell me about a time when a customer was reporting a technical issue that you didn’t know the answer to. What was your approach, and how did it end up?
If you have experienced this, then share your experience with the interviewer truthfully.
If not, then remember the basic rule: Never say, “I don’t know.” Instead, you can say I will get back to you with the information you have requested. Then talk to your supervisors and get the solution; otherwise, you can redirect the call to them.