Customer cannont reach a specific phone number

I am troubleshooting an issue with a customer on our hosted freepbx solution. They are unable to call a specific number that they could before. We attempted to contact the number from a cell phone and were successful. I have checked all configs in the xorcom and they are correct. What else could cause this??

If all other numbers work, you will have to check with your service provider.

My company is providing the service.