If I can’t solve the problem, I:

Always give options when you are not able to give them what they asked for. Try to keep bureaucratic stuff away and provide the information they need in a way they will understand. Also, try to keep a positive tone during the call and choose upbeat words, such as: "I'll be glad to," "I'm happy to." Try not to use negative words and expressions like "that's not possible" or "no can do" because it may bother them. Check out this fascinating website that talks about how to sound happy on the phone: https://www.ruby.com/how-to-sound-happy-on-the-phone-and-make-your-callers-happier-in-the-process/

Photo by Joyce Busola on Unsplash: https://unsplash.com/photos/L9U5UUScnHY