Most carriers will identify the porting information based on the service address information, not the billing address. Since a bill copy is a billing record, it may not provide the same information as the service address for the telephone number(s) being ported. The only exceptions to this are toll-free and wireless numbers because they're virtual numbers and therefore have no service address.
For a list of other common rejections and resolutions, check out the Common Porting Rejections and Resolutions section.