The best way to answer this question is to first apologize to the client for not knowing the answer to their question.
Then, explain that you will do your best to find the answer and provide them with the most up-to-date information.
Then, offer to put them on hold or transfer the call to someone who may be better able to help them.
Finally, thank the client for their patience and let them know that you appreciate them taking the time to contact you.