What would you say when you can’t resolve the customer's issue on the phone?

Ask some time to do research so you'll get breathing time 

Issues of a customer that cannot be resolved on the phone needs an appointment where you will meet the customer and listen to him or her fully.  Then the customer complaint form needs to be filled up with the complaint logged in the company log in book.  The next person in authority will look into the complaint and decide the solution to the issue in a written form can be email or letter with an apology and also giving a one off discount to the customer if need be (if that would make him a happier person) and the company in this way would retain the customer.  If it is a staff problem with the customer then the person can be warned to better conduct on the company premises or manners maintained on the phone at all times.

In my experience, I honestly inform my customer what can be done immediately and what will take more time. While providing a Turn-Around-Time and making sure to get the issue resolved for him within that time of simply dial a follow-up call to let them know we are working on it and it may need more time.

Agreed with the answers of colleagues

Thank you, I politely regret uable to respond to his quires at the moment and ask him to contact in the nearst time , and promise that meanwhile will arrange to resolve the issue.

Regards .

Try to get an appointment for face to face meeting.

if cant resolve by the phone try personaly,anyway that is common for everyone,theres essue needs eye to contact meeting @ giving some ideas possible

book an appointment with the customer so that you help resolve the issue on a one on one basis

I will inform him that this is beyond my control and politely and honestly inform the customer that you need to transfer him to the person who is best suited to answer his inquiry. 

Set up a meeting to better explain and understand for effective response.