You need to call your long distance carrier and tell them the toll free number you are calling about, tell them that it is ringing busy, they will want to know if it's a regular busy or a fast busy. They will also probably require a "call example" A call example is the exact date, time calling from, calling to number and the result. Make sure to tell them the time zone you are in. It's always best to have a "fresh" call example meaning a recent one that just happend. Ask your customer to call the number again and then make sure you find out exactly what number they called it from. Tell your LD carrier everything you can about the problem and they should be able to tell you if they are having an outage or if it's on your end.
You may also want to try to make some test calls yourself.
Good luck!
Toni