Explain that you do not understand what he's saying, is it another panguage or an accent?
Pass his call to someone else who may have a better time understanding him, or ask if there is someone on HIS end that would be able to explain the issue to you because you do not have much experience understanding his accent.
Preferably, just have someone else field his call and take the blame upon your company, no this speech. I used to just say that my telephone had a really bad line, that gets them to speak up and speak slower, or request an email with the details.
"Sir, you have a good point, and one that I want to get a definitive answer to. Would you be able to send me an email with the details so that our team can investigate the best solution for you once and for all and get right back to you?"
Now he feels important and clever, he's stumped you, but you get it on paper and resolve his issue. Now you are a great team that works hard to find the right solution.