Do you have the ability to look at the incoming call setup information for incoming calls?
If so, do you see a call setup coming in?
If so, look at the called party number - possibly the number of digits sent does not match what is programmed in the customers equipment.
You might try calling the number in question from an ISDN line and log the ISDN protocol on that dialing line. You should be able to get a disconnect cause and location that will help narrow things down (you may be able to do that with a another VOIP line instead of ISDN).
Best of luck