By the sounds of it, I would expect a customization to be the issue here.
Can you confirm the exact steps you take, when creating a new customer record?
Usually you can control with security, if users are able to create a new customer record, but then it will not be possible for the users to create the customer data at all. The "New" button would generally be unavailable, if users are not granted access to do so.
Then there could be specific table/field level access defined on top, but that usually throws another type of error message, like "Access denied.."
The role "Inside Sales Agent" seems like a custom role, please check the contents of that role.
Have any changes been made to the standard roles/duties or privileges?