In ARS, the system can add or remove digits. Usually the ARS tables are designed around a type of call, for example, 1-800 goes one route, 1-408 goes another route, 976-xxxx goes nowhere, 1-900 goes nowhere, etc.
So let's say that he's calling 1-202-777-1414. Do other 1+ calls go through? do other calls to 1-202- go through? Do other calls to 1-202-777-xxxx go through? If other calls fail, check the ARS for that dialing string.
If all other calls go through, and only the one business can be reached by cell but not by your system, then it might not be ARS. It might be that your telco has not updated it's routing, and the number you're trying to reach recently changed telcos and had their numbers "ported over."
If you have copper trunks -- plain old phone lines like in a house -- you can test this by calling from the place where they enter the building (the MPOE). If it works at the MPOE, the problem is in your switch and its ARS, and if not, then the problem is at the telco.
If the problem is at the telco, call repair and open a trouble ticket.