I should have added something. Backup withholding (BUW) occurs when you don't furnish the proper EIN or SSN (in the case of individual) to the payer. They are supposed to request it on form w-9, and you are supposed to fill it out and send it back - use the EIN of the S-Corp if that's the proper recipient otherwise use your SSN for non-business items.
BUW can also occur if you have properly filed your returns and paid the taxes due.
