Withholding Tax - Notifying Vendors of Withheld Amounts

The big question that we have regarding withholding tax is how other Oracle clients notify the supplier that amounts have been withheld on their invoices.  Have they had to modify their check process to notify suppliers of what was withheld since the w/h amount does not appear on the check remittance (it would be a customization to include it on the check)?  Do they send a form letter with the check notifying the vendor that amounts were withheld?  We are trying to avoid the calls from suppliers to Accounts Payable.

Would appreciate any insight!