It is important to remain calm, polite and respectful towards the client notwithstanding the difficulty of the situation and your frustration, and to always act as a professional representative of the company.
Tell the interviewer that you know you ought to answer clients to the best of your ability, and should you be unable to completely resolve the situation, the best course of action is to forward this client to a superior.
"While working at a call center during my undergraduate studies, a client called me when I was working overtime and my superior was out of the office. He was very frustrated because he couldn't access his emails through his company portal. I did my best to listen to him, let him vent and acknowledge his problem. I helped him to the best of my abilities, based upon the training I had received, and when I realized I was not in a position to resolve his issue, I tried to reach out to some of my colleagues to see whether they had encountered such issue. Ultimately, I explained to the client a supervisor would be in touch with him first thing Monday morning, and provided my extension for him to contact me should he not hear back in a timely fashion. I always ensure to remain respectful even if the clients are extremely frustrated and to take their problems seriously. I also always follow up to ensure their issues have been resolved and that they are satisfied with the services received."